Propulsion system control

ABSTRACT

A method of operating an aircraft that includes a propulsion system. The propulsion system includes a gas turbine engine and a fuel tank arranged to provide fuel to the gas turbine engine. The method includes: determining at least one fuel characteristic of the fuel arranged to be provided to the gas turbine engine; and controlling the propulsion system based on the at least one fuel characteristic.

CROSS-REFERNCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Application Serial No.17/853,291, filed on 29^(th) June 2022, which claims priority fromUnited Kingdom Patent Application Number 2118653.1, filed on 21^(st)December 2021. The entire contents of each of these applications areincorporated herein by reference in their entirety.

BACKGROUND Field of the Disclosure

The present disclosure relates to aircraft propulsion systems, and tomethods of operating aircraft involving the management of fuels ofdifferent types, including detection of fuel properties and actionstaken to improve aircraft performance based on the data acquired, and tomethods of modifying aircraft so as to allow such methods to beimplemented.

Description of the Related Art

There is an expectation in the aviation industry of a trend towards theuse of fuels different from the traditional kerosene-based jet fuelsgenerally used at present. These fuels may have differing fuelcharacteristics, for example having either or both of a lower aromaticcontent and lower sulphur content, relative to petroleum-basedhydrocarbon fuels.

Thus, there is a need to take account of fuel properties in light of theincreased possibility of variation, and to adjust the control andmanagement of aircraft propulsion systems and fuel supplies for thesenew fuels.

SUMMARY

According to a first aspect there is provided a method of identifying afuel contained in a fuel tank of an aircraft and arranged to power a gasturbine engine of the aircraft, the method being performed by processingcircuitry of the aircraft and comprising:

-   obtaining one or more fuel characteristics of any fuel already    present in the fuel tank prior to refuelling;-   determining one or more fuel characteristics of a fuel added to the    fuel tank on refuelling; and-   calculating one or more fuel characteristics of the resultant fuel    in the fuel tank after refuelling.

This approach may be referred to as an active infinite summing approach,as action is taken to make and continuously update a record of the fuelonboard an aircraft. The method may be performed for each of multiplefuel tanks of the aircraft separately, or for the overall fuel onboardthe aircraft irrespective of in which tank it is held.

The obtaining the one or more fuel characteristics of any fuel alreadypresent in the fuel tank prior to refuelling may comprise obtainingthose characteristics from computational storage, detecting thosecharacteristics directly, or determining those characteristics fromother detected parameters.

The step of obtaining one or more fuel characteristics of any fuelalready present in the fuel tank prior to refuelling may comprisedetecting one or more features of the composition of the fuel alreadypresent in the fuel tank.

The step of obtaining one or more fuel characteristics of any fuelalready present in the fuel tank prior to refuelling may compriseobtaining the result of an earlier determination performed using themethod of identifying a fuel described above for this first aspect.

The step of obtaining one or more fuel characteristics of any fuelalready present in the fuel tank prior to refuelling may comprisemaintaining current fuel characteristic data by updating the fuelcharacteristics of the fuel present in the fuel tank following eachrefuelling of the aircraft.

The step of determining one or more fuel characteristics of the fueladded to the fuel tank on refuelling may comprise reading a barcodeassociated with the provided fuel.

The fuel characteristics may be or comprise parameters of a hydrocarbondistribution of the fuel. The fuel characteristics may be or comprise:

-   i. the percentage of sustainable aviation fuel in the fuel;-   ii. the aromatic hydrocarbon content of the fuel;-   iii. the multi-aromatic hydrocarbon content of the fuel;-   iv. the percentage of nitrogen-containing species in the fuel;-   v. the presence or percentage of a tracer species or trace element    in the fuel (e.g. a trace substance inherently present in the fuel    which may vary between fuels and so be used to identify a fuel,    and/or a substance added deliberately to act as a tracer);-   vi. the hydrogen to carbon ratio (H/C) of the fuel;-   vii. the hydrocarbon distribution of the fuel;-   viii. level of non-volatile particulate matter (nvPM) emissions on    combustion (e.g. on combustion for a given combustor design, at a    given operating condition);-   ix. naphthalene content of the fuel;-   x. sulphur content of the fuel;-   xi. cycloparaffin content of the fuel;-   xii. oxygen content of the fuel;-   xiii. thermal stability of the fuel (e.g. thermal breakdown    temperature);-   xiv. level of coking of the fuel;-   xv. an indication that the fuel is a fossil fuel, for example fossil    kerosene; and-   xvi. one or more properties such as density, viscosity, calorific    value, and/or heat capacity.

The method may further comprise chemically or physically detecting oneor more parameters of the resultant fuel in the fuel tank afterrefuelling, and verifying one or more of the calculated fuelcharacteristics based on the one or more detected parameters. Thedetected parameters may be fuel characteristics, or may be used tocalculate or infer fuel characteristics - for example, the detectedparameters may be shaft speed and mass flow rate of fuel, from whichcalorific value (a fuel characteristic) may be determined, or thedetected parameters may be fuel density and/or the presence of a tracer,both of which are fuel characteristics.

The obtaining the one or more fuel characteristics of any fuel alreadypresent in the fuel tank prior to refuelling may comprise obtainingstored fuel characteristic data. The method may further comprisechemically or physically detecting one or more parameters of any fuelalready present in the fuel tank prior to refuelling, and verifying theinput to the calculating step based on the one or more detectedparameters.

The method may further comprise chemically and/or physically determiningone or more parameters of the fuel in the fuel tank, and using thedetermined values to replace the stored fuel characteristics for thefuel in the fuel tank.

The chemically and/or physically determining one or more parameters ofthe fuel in the fuel tank may be performed by extracting a sample of thefuel from the fuel tank for off-wing testing.

The chemically and/or physically determining one or more parameters ofthe fuel in the fuel tank and using the determined values to replace thestored fuel characteristics for the fuel in the fuel tank may beperformed in response to a trigger event, such as:

-   i. a threshold amount of time having elapsed since a previous    chemical and/or physical determination of the one or more parameters    of the fuel in the fuel tank;-   ii. a threshold number of refuelling events and/or flights having    been reached since a previous determination of the one or more    parameters of the fuel in the fuel tank; and/or-   iii. a discrepancy between one or more of the calculated    characteristics and a detected parameter exceeding a threshold.

The method may further comprise controlling the propulsion system basedon the calculated one or more fuel characteristics of the resultant fuelin the fuel tank after refuelling, for example as described below withrespect to the fourth and fifth aspects.

The method may further comprise proposing or initiating a change to theflight profile based on the one or more fuel characteristics of theresultant fuel in the fuel tank after refuelling, for example asdescribed below with respect to the sixth and seventh aspects.

According to a second aspect, there is provided a method of controllingthe propulsion system of an aircraft, the propulsion system comprising agas turbine engine and a fuel tank arranged to provide fuel to the gasturbine engine, the method comprising:

-   obtaining one or more fuel characteristics of any fuel already    present in the fuel tank prior to refuelling;-   determining one or more fuel characteristics of a fuel added to the    fuel tank on refuelling;-   calculating one or more fuel characteristics of the resultant fuel    in the fuel tank after refuelling; and-   controlling the propulsion system based on the calculated one or    more fuel characteristics of the resultant fuel in the fuel tank    after refuelling.

The obtaining one or more fuel characteristics of any fuel alreadypresent in the fuel tank prior to refuelling may comprise:

-   (i) detecting one or more features of the composition of the fuel    already present in the fuel tank; or-   (ii) obtaining the result of an earlier determination performed    using the method of the first aspect.

According to a third aspect, there is provided a propulsion system foran aircraft comprising:

-   a gas turbine engine, the gas turbine engine optionally comprising:    -   an engine core comprising a turbine, a compressor, and a core        shaft connecting the turbine to the compressor; and    -   a fan located upstream of the engine core, the fan comprising a        plurality of fan blades and being arranged to be driven by an        output from the core shaft;-   a fuel tank arranged to contain a fuel to power the gas turbine    engine; and-   a fuel composition tracker arranged to:    -   store current fuel characteristic data, the fuel characteristic        data comprising one or more fuel characteristics of fuel present        in the fuel tank;    -   obtain one or more fuel characteristics of a fuel added to the        fuel tank on refuelling; and    -   calculate updated values for the one or more fuel        characteristics of the fuel in the fuel tank after refuelling.

The updated values may then take the place of the stored values, for usein future iterations of the steps performed by the fuel compositiontracker.

The fuel characteristic data may be fuel composition data, including oneor more parameters of a hydrocarbon distribution of the fuel.

According to a further aspect, there is provided a non-transitorycomputer readable medium having stored thereon instructions that, whenexecuted by a processor, cause the processor to perform the method ofthe first and/or second aspects. The processor may be, or may be a partof, an Electronic Engine Controller of the aircraft.

According to a fourth aspect, there is provided a method of operating anaircraft comprising a gas turbine engine and a fuel tank arranged toprovide fuel to the gas turbine engine, the method comprising:

-   determining one or more fuel characteristics of the fuel arranged to    be provided to the gas turbine engine; and-   proposing or initiating a change to a flight profile of the aircraft    based on the one or more fuel characteristics.

Implementations of this aspect may therefore allow environmentalbenefits (e.g. reduced or tailored contrail formation) and/oroperational benefits (e.g. improved fuel burn efficiency) to be obtainedbased on knowledge of the fuel being burned.

In some examples, the method may comprise automatically initiating thechange to the flight profile based on the determined characteristics. Insome examples, the method may comprise notifying a pilot of thesuggested change to the flight profile based on the determinedcharacteristics and allowing the pilot the opportunity to confirm orcancel the change. In some implementations, either example may beimplemented depending on the nature of the change.

The one or more fuel characteristics of the fuel may comprise at leastone of the fuel characteristics as listed above.

The change to the flight profile based on the fuel characteristics maycomprise at least one of:

-   (i) a change to the intended altitude; and-   (ii) a change to the intended route.

The determining one or more fuel characteristics of the fuel maycomprise implementing the method of the first aspect, in particular by:

-   obtaining one or more fuel characteristics of any fuel already    present in the fuel tank prior to refuelling;-   determining one or more fuel characteristics of a fuel added to the    fuel tank on refuelling; and-   calculating one or more fuel characteristics of the resultant fuel    in the fuel tank after refuelling.

The method may further comprise receiving forecast weather conditionsfor an intended route of the aircraft. The received forecast weatherconditions may be used to influence the changes in planned route and/oraltitude.

The determining the one or more fuel characteristics may be performedbased on detection of one or more fuel properties. The detection may beperformed on-wing.

The determining the one or more fuel characteristics may be performedbased on received fuel composition data, e.g. data sent electronicallyto the aircraft by a third party, or entered using a user interfaceonboard the aircraft. The fuel composition data may be provided to theaircraft on refuelling.

The one or more fuel characteristics may be determined for fuel in oneor more fuel tanks of the aircraft.

One or more of the fuel characteristics, e.g. calorific value, may beinferred from performance of the gas turbine engine during at least oneof engine warm-up, taxi, take-off and climb of the aircraft. The plannedflight profile during cruise may be changed / the flight profile may beupdated based on the one or more inferred fuel characteristics.

According to a fifth aspect, there is provided a propulsion system foran aircraft comprising:

-   a gas turbine engine, the gas turbine engine optionally comprising:    -   an engine core comprising a turbine, a compressor, and a core        shaft connecting the turbine to the compressor; and    -   a fan located upstream of the engine core, the fan comprising a        plurality of fan blades and being arranged to be driven by an        output from the core shaft;-   a fuel tank arranged to contain a fuel to power the gas turbine    engine; and a fuel composition determination module arranged to:    -   determine one or more fuel characteristics of the fuel arranged        to be provided to the gas turbine engine; and-   a flight profile adjustor arranged to:    -   propose or initiate a change to a flight profile of the aircraft        based on the one or more fuel characteristics of the fuel.

The flight profile adjustor may be arranged to initiate or propose atleast one of the following based on the fuel characteristics:

-   (i) a change to the intended altitude; and-   (ii) a change to the intended route.

The propulsion system may be arranged to perform the method as describedwith respect to the fourth aspect.

According to a sixth aspect, there is provided a method of operating anaircraft comprising a propulsion system, the propulsion systemcomprising a gas turbine engine and a fuel tank arranged to provide fuelto the gas turbine engine, the method comprising:

-   determining one or more fuel characteristics of the fuel arranged to    be provided to the gas turbine engine; and-   controlling the propulsion system based on the one or more fuel    characteristics.

Implementations of this aspect may therefore allow environmentalbenefits (e.g. reduced or tailored contrail formation) and/oroperational benefits (e.g. improved fuel burn efficiency) to be obtainedbased on knowledge of the fuel being burned.

In some examples, the method may comprise controlling the propulsionsystem based on the determined characteristics, without seeking pilotinput or approval. In some examples, the method may comprise notifying apilot of the suggested change to the control of the propulsion system,based on the determined characteristics and allowing the pilot theopportunity to confirm or cancel the change. In some implementations,either example may be implemented depending on the nature of the change.The controlling may therefore be implemented directly, or afterverification.

The method may be performed iteratively in flight, e.g. due to changesin fuel supplied to the gas turbine engine and/or changes in conditionsand flight stage.

The one or more fuel characteristics of the fuel may comprise one ormore of the characteristics listed above.

The method may further comprise receiving weather data relating toweather conditions around the aircraft or along a planned route of theaircraft. The received weather data may be used to influence the controlof the propulsion system.

The method may further comprise detecting weather conditions around theaircraft in flight. The detected weather conditions may be used toinfluence the control of the propulsion system.

The control of the propulsion system based on the fuel characteristicsmay comprise making changes to one or more of the following in flight:

-   An operating parameter of a heat management system of the aircraft    (e.g. a fuel-oil heat exchanger) may be changed, or the temperature    of fuel supplied to a combustor of the engine may be changed.-   When more than one fuel is stored aboard an aircraft, a selection of    which fuel to use for which operations (e.g. for ground-based    operations as opposed to flight, for low-temperature start-up, or    for operations with different thrust demands) may be made based on    fuel characteristics such as %Sustainable Aviation Fuel (SAF),    non-volatile Particulate Matter (nvPM) generation potential,    viscosity, and calorific value. A fuel delivery system may therefore    be controlled appropriately based on the fuel characteristics.-   One or more flight control surfaces of the aircraft may be adjusted    so as to change route and/or altitude based on knowledge of the    fuel.-   The spill percentage of a fuel pump (i.e. the proportion of pumped    fuel recirculated instead of being passed to the combustor) based on    the %SAF of the fuel. The pump and/or one or more valves may    therefore be controlled appropriately based on the fuel    characteristics.-   Changes to the scheduling of variable-inlet guide vanes (VIGVs) may    be made based on fuel characteristics. The VIGVs may therefore be    moved, or a movement of the VIGVs be cancelled, as appropriate based    on the fuel characteristics.

These options may be referred to as control examples, as they areexamples of ways in which the propulsion system may be controlled basedon fuel characteristics.

(The sustainable aviation fuel percentage (%SAF) of a fuel may begravimetric or volumetric – it will be appreciated that there are often– generally small – differences in density between SAFs and traditionaljet fuels such as Jet A.)

The determining the one or more fuel characteristics may be performedbased on detection of one or more fuel properties. The detection may beperformed on-wing.

The determining the one or more fuel characteristics may be performedbased on received fuel composition data. The fuel composition data maybe provided to the aircraft on refuelling.

The one or more fuel characteristics may be determined for fuel in oneor more fuel tanks of the aircraft.

The one or more fuel characteristics may be determined for each of aplurality of fuels stored onboard the aircraft.

The one or more fuel characteristics may be determined for fuelimmediately before entry into a combustor of the gas turbine engine. Thedetermination of the one or more fuel characteristics of fuelimmediately before entry into a combustor of the gas turbine engine maybe performed a plurality of times during flight to account for changesin fuel composition.

The one or more of the fuel characteristics may be inferred fromperformance of the gas turbine engine during at least one of enginewarm-up, taxi, take-off, and climb of the aircraft. The propulsionsystem may be controlled during cruise based on the one or more inferredfuel characteristics.

The determining the one or more fuel characteristics of the fuel maycomprises implementing the method of the first aspect, in particular:

-   obtaining one or more fuel characteristics of any fuel already    present in the fuel tank prior to refuelling;-   determining one or more fuel characteristics of a fuel added to the    fuel tank on refuelling; and-   calculating one or more fuel characteristics of the resultant fuel    in the fuel tank after refuelling.

According to a seventh aspect, there is provided a propulsion system foran aircraft comprising:

-   a gas turbine engine, the gas turbine engine optionally comprising    an engine core comprising a turbine, a compressor, and a core shaft    connecting the turbine to the compressor; and a fan located upstream    of the engine core, the fan comprising a plurality of fan blades and    being arranged to be driven by an output from the core shaft;-   a fuel tank arranged to contain a fuel to power the gas turbine    engine;-   a fuel composition determination module arranged to determine one or    more fuel characteristics of the fuel arranged to be provided to the    gas turbine engine; and-   a propulsion system controller arranged to control the propulsion    system based on the one or more fuel characteristics of the fuel.

The fuel composition determination module may comprise a receiverarranged to receive data relating to fuel composition. The fuelcomposition determination module may be arranged to determine one ormore fuel characteristics based on the received data.

The fuel composition determination module may comprise one or moresensors arranged to provide data relating to one or more fuelcharacteristics. The fuel composition determination module may bearranged to determine one or more fuel characteristics based on thesensor data.

The propulsion system may comprise a plurality of fuel tanks arranged tocontain different fuels to power the gas turbine engine. The fuelcomposition determination module may be arranged to determine at leastone fuel characteristic of each different fuel.

The propulsion system may be arranged to perform the method as describedwith respect to the sixth aspect.

According to an eighth aspect, there is provided a propulsion system foran aircraft comprising:

-   a gas turbine engine, the gas turbine engine optionally comprising:    -   an engine core comprising a turbine, a compressor, and a core        shaft connecting the turbine to the compressor; and    -   a fan located upstream of the engine core, the fan comprising a        plurality of fan blades and being arranged to be driven by an        output from the core shaft;-   a plurality of fuel tanks, each arranged to contain a different fuel    to be used to power the gas turbine engine, wherein the fuels have    different calorific values; and-   a fuel manager arranged to store information on the fuel contained    in each fuel tank and to control fuel input to the gas turbine    engine in operation (optionally in flight only) by selection of a    specific fuel or fuel combination from one or more of the plurality    of fuel tanks based on thrust demand of the gas turbine engine such    that a fuel with a lower calorific value is supplied to the gas    turbine engine at lower thrust demand.

It will be appreciated that the propulsion system may compriseadditional fuel tanks containing the same fuels in addition to aplurality of fuel tanks containing different fuels; a minimum of twodifferent fuels is provided onboard the aircraft for implementations ofthis aspect.

The variation in calorific value of the fuel corresponding to thrustdemand may facilitate maintenance of a more constant fuel flow rateduring at least one section of a flight (e.g. at cruise, or for aconstant altitude section of cruise), and/or more even fuel pump andspill operation in flight during at least one section of a flight.Further, a lowest necessary fuel flow rate at key points (e.g. very lowthrust points of operation) may be increased by moving to a lowercalorific value fuel, so raising an overall minimum flow level andkeeping flow within a narrower range across the entire flight envelope.

Implementations of this aspect may therefore allow a higher fuel massflow rate to be maintained at lower thrust demand than if a fuel werenot selected based on calorific value, so doing one or more offacilitating use of the fuel as a heat transfer fluid (provided that thelower calorific value fuel in question does not have a correspondinglylower heat capacity), improving lubrication, and/or reducing the chanceof fuel overheating. This may be of particular utility when running atlow idle thrust. Similarly, the use of a fuel with a higher calorificvalue at higher thrust demand may facilitate meeting that demand withoutstressing the fuel flow management system. Implementation of the presentaspect may therefore mean that, when thrust demand is reduced, the fuelflow rate does not have to be reduced as far as it would otherwise haveto be reduced.

Each fuel tank may be arranged to contain a fuel with a different typeor proportion of a sustainable aviation fuel.

A first fuel tank of the plurality of fuel tanks may be arranged tocontain only a fuel which is a sustainable aviation fuel. Thesustainable aviation fuel in the first fuel tank may be selected suchthat the propulsion system can be run on that fuel alone.

The fuel manager may be arranged to implement different control forground-based operations as compared to flight. For example, sustainableaviation fuel in the first fuel tank, or a high %SAF blend, may be usedto power the aircraft when the aircraft is performing at least themajority of operations on the ground, irrespective of thrust demand orof calorific value of that fuel.

The fuel manager may be arranged such that all fuel used for groundoperations is taken from the first fuel tank, and/or such that all fuelused for ground operations is SAF or the highest %SAF blend available tothe aircraft.

The fuel manager may be arranged such that a fuel with a lower calorificvalue is supplied to the gas turbine engine at cruise than during climb.

The fuel manager may be arranged such that a fuel with a lower calorificvalue is supplied to the gas turbine engine at low idle than at highidle.

A first fuel tank of the plurality of fuel tanks may have a higherproportion of sustainable aviation fuel (e.g. 100%) than a second fueltank of the plurality of fuel tanks. In some cases, more fuel from thesecond fuel tank may be used at cruise and more fuel from the first tankused at operating points with higher power demands. The higher %SAF fuelmay have a higher calorific value.

First and second fuel tanks of the plurality of fuel tanks may containsustainable aviation fuels of different compositions.

The fan may have a diameter of at least 330 cm.

According to a ninth aspect, there is provided a method of operating anaircraft comprising a gas turbine engine and a plurality of fuel tanksarranged to store fuel to power the gas turbine engine, the methodcomprising:

-   arranging each fuel tank of the plurality of fuel tanks to contain a    different fuel to be used to power the gas turbine engine, wherein    the fuels have different calorific values;-   storing information on the fuel contained in each fuel tank; and-   controlling fuel input to the gas turbine engine in operation    (optionally in flight only) by selection of a specific fuel or fuel    combination from one or more of the plurality of fuel tanks based on    thrust demand of the gas turbine engine such that a fuel with a    lower calorific value is supplied to the gas turbine engine at lower    thrust demand.

The arranging each fuel tank to contain a different fuel may comprisesupplying each fuel tank with a different sustainable aviation fuel,and/or with a blended fuel with a different type or proportion of asustainable aviation fuel.

The controlling fuel input to the gas turbine engine based on thrustdemand may be performed only in flight. Fuel input may be differentlycontrolled for ground-based operations.

According to a tenth aspect, there is provided a method of modifying anaircraft comprising a gas turbine engine and a plurality of fuel tanksarranged to store fuel to power the gas turbine engine, the methodcomprising:

-   arranging each fuel tank to contain a different fuel to be used to    power the gas turbine engine, wherein the fuels have different    calorific values; and-   providing a fuel manager arranged to store information on the fuel    contained in each fuel tank and to control fuel input to the gas    turbine engine in operation (optionally in flight only) by selection    of a specific fuel or fuel combination from one or more of the    plurality of fuel tanks based on thrust demand of the gas turbine    engine such that a fuel with a lower calorific value is supplied to    the gas turbine engine at lower thrust demand.

The arranging each fuel tank to contain a different fuel may comprisesupplying each fuel tank with a different sustainable aviation fuel,and/or with a blended fuel with a different type or proportion of asustainable aviation fuel.

The arranging each fuel tank to contain a different fuel may compriseadjusting at least one valve so as to fluidly isolate two or more fueltanks from each other so as to provide separate containment fordifferent fuels.

The fuel manager may be arranged to implement different control forground-based operations as compared to flight. The control of fuel inputto the gas turbine engine based on thrust demand may be performed onlyin flight. Fuel input may therefore be differently controlled forground-based operations, e.g. selecting SAF (or a higher %SAF blend)irrespective of calorific value if the choice is between that and afossil-based fuel (or a lower %SAF blend).

According to an eleventh aspect, there is provided a power system for anaircraft comprising:

-   a gas turbine engine arranged to burn a fuel so as to provide power    to the aircraft;-   a plurality of fuel tanks, each arranged to contain a fuel to be    used to provide power to the aircraft, wherein at least two tanks of    the plurality of fuel tanks contain different fuels, and wherein one    or more tanks of the plurality of fuel tanks are arranged to contain    only a fuel which is a sustainable aviation fuel; and-   a fuel manager arranged to store information on the fuel contained    in each fuel tank and to control fuel supply so as to take only the    sustainable aviation fuel to power at least the majority of    operations on the ground.

In some examples, only sustainable aviation fuel may be used to poweraircraft operations on the ground, such that all ground-based operationsare powered using sustainable aviation fuel.

In other examples, most but not all of the fuel used for groundoperations is sustainable aviation fuel, with only small amounts fromother sources being used (e.g. less than 10% or less than 5% of the fueluse and/or of the operation time of ground-based operations).

In some examples, especially in examples wherein the sustainableaviation fuel has a higher viscosity at a given temperature than thefuel in another fuel tank, fuel from the another fuel tank may be usedfor start-up of the engine, and the fuel source may then be switched tothe sustainable aviation fuel once the engine is running, e.g. once acertain temperature has been reached. The fuel in the tank used forstart-up may be optimised for low-temperature initial use, and/or forother features of start-up operation. In such examples, sustainableaviation fuel may be used for all ground-based operations except forengine start-up if the fuel in the start-up tank is not also SAF, andSAF (optionally different SAFs) may be used for all ground-basedoperations if it is.

A gas turbine engine of the one or more gas turbine engines may be a gasturbine engine of an Auxiliary Power Unit – APU. The APU may be arrangedto be active primarily or only during ground-based operations.

A first fuel tank of the one or more tanks may be arranged to contain asustainable aviation fuel and may be exclusively dedicated to the APUsuch that the sustainable aviation fuel from the first fuel tank is notarranged to be provided to any other gas turbine engine of the aircraft.A fuel not certified for use to power flight may therefore be stored inthe first fuel tank.

A first fuel tank of the one or more tanks may be arranged to contain asustainable aviation fuel, and may be arranged to provide fuel to theAPU when performing operations on the ground, and to serve as a trimtank in flight.

The APU may not be arranged to provide any propulsive power to theaircraft.

The gas turbine engine may be arranged to provide propulsive power tothe aircraft, and may comprise:

-   an engine core comprising a turbine, a compressor, and a core shaft    connecting the turbine to the compressor; and-   a fan located upstream of the engine core, the fan comprising a    plurality of fan blades and being arranged to be driven by an output    from the core shaft.

According to a twelfth aspect, there is a provided a propulsion systemfor an aircraft comprising:

-   a gas turbine engine, the gas turbine engine optionally comprising:    -   an engine core comprising a turbine, a compressor, and a core        shaft connecting the turbine to the compressor; and    -   a fan located upstream of the engine core, the fan comprising a        plurality of fan blades and being arranged to be driven by an        output from the core shaft;-   a plurality of fuel tanks, each arranged to contain a fuel to be    used to provide power to the aircraft, wherein at least two tanks of    the plurality of fuel tanks contain different fuels, and wherein one    or more tanks of the plurality of fuel tanks are arranged to contain    only a fuel which is a sustainable aviation fuel; and-   a fuel manager arranged to store information on the fuel contained    in each fuel tank and to control fuel input to the gas turbine    engine so as to use only the sustainable aviation fuel when the    aircraft is performing at least the majority of operations on the    ground.

Each fuel tank may be arranged to contain a fuel with a different typeof sustainable aviation fuel and/or a different proportion of asustainable aviation fuel. In some implementations, two or more tanksmay contain the same fuel.

The sustainable aviation fuel in at least a first fuel tank of the oneor more tanks arranged to contain a sustainable aviation fuel may beselected such that the propulsion system can be run on that fuel alone.

The fuel manager may be arranged to control fuel input to the gasturbine engine in flight by selection of a specific fuel or fuelcombination from one or more of the plurality of fuel tanks.

The fuel in the one or more tanks arranged to contain a sustainableaviation fuel for use in ground-based operations may have a lowercalorific value than any fuel stored in another fuel tank of theplurality of fuel tanks. The fuel in the one or more tanks arranged tocontain a sustainable aviation fuel for use in ground-based operationsmay give lower nvPM emissions than any fuel stored in another fuel tankof the plurality of fuel tanks. The fuel selected for use forground-based operations may be optimised for ground-based operations,some or all of which may have a relatively low power demand as comparedto average in-flight operation, and some or all of which may be requiredby regulations to meet more stringent emissions criteria.

A first fuel tank of the one or more tanks arranged to contain asustainable aviation fuel may be smaller than the one or more other fueltanks. The first fuel tank may be arranged to be used exclusively forground-based operations of the aircraft. This arrangement may be asdescribed with respect to the sixteenth to twentieth aspects, below. Thefuel in the first fuel tank may be selected to have a lower calorificvalue than any fuel stored in another fuel tank of the plurality of fueltanks.

The sustainable aviation fuel may be used to power all ground-basedoperations of the aircraft.

According to a thirteenth aspect, there is a provided a method ofoperating an aircraft comprising a gas turbine engine and a plurality offuel tanks arranged to store fuel to power the gas turbine engine, themethod comprising:

-   arranging at least two fuel tanks of the plurality of fuel tanks to    store different fuels, wherein one or more tanks of the plurality of    fuel tanks are arranged to contain only a fuel which is a    sustainable aviation fuel;-   controlling fuel supply so as to use only the sustainable aviation    fuel when the aircraft is performing at least the majority of    operations on the ground.

The method may further comprise storing information on the fuelcontained in each fuel tank. The control may be performed based on thestored information.

The gas turbine engine may be arranged to provide propulsive power tothe aircraft, and may comprise:

-   an engine core comprising a turbine, a compressor, and a core shaft    connecting the turbine to the compressor; and-   a fan located upstream of the engine core, the fan comprising a    plurality of fan blades and being arranged to be driven by an output    from the core shaft.

The gas turbine engine may be a gas turbine engine of an Auxiliary PowerUnit – APU – of the aircraft.

A first fuel tank of the one or more tanks arranged to contain asustainable aviation fuel may be a trim tank of the aircraft. Thesustainable aviation fuel in the first fuel tank may be arranged to be(at least substantially) used up performing the operations on the groundsuch that the first fuel tank is at least substantially empty andavailable to receive fuel pumped thereinto in flight.

According to a fourteenth aspect, there is provided a method ofmodifying an aircraft comprising a gas turbine engine and a plurality offuel tanks arranged to store fuel to power the gas turbine engine, themethod comprising:

-   arranging at least two fuel tanks of the plurality of fuel tanks to    each store a different fuel, wherein one or more tanks of the    plurality of fuel tanks are arranged to contain only a fuel which is    a sustainable aviation fuel; and-   providing a fuel manager arranged to control fuel supply so as to    use only the sustainable aviation fuel when the aircraft is    performing at least the majority of operations on the ground.

The fuel manager may be arranged to store information on the fuelcontained in each fuel tank. The fuel manager may be arranged to performthe control of the fuel supply based on the stored information.

The gas turbine engine may be arranged to provide propulsive power tothe aircraft, and may comprise:

-   an engine core comprising a turbine, a compressor, and a core shaft    connecting the turbine to the compressor; and-   a fan located upstream of the engine core, the fan comprising a    plurality of fan blades and being arranged to be driven by an output    from the core shaft.

The gas turbine engine may be a gas turbine engine of an Auxiliary PowerUnit – APU – of the aircraft.

According to a fifteenth aspect, there is provided a power system for anaircraft comprising:

-   an Auxiliary Power Unit – APU – comprising a gas turbine engine    arranged to burn a fuel so as to provide power to the aircraft; and-   one or more fuel tanks arranged to contain only a fuel which is a    sustainable aviation fuel;-   and wherein all fuel used by the APU is the sustainable aviation    fuel.

According to a sixteenth aspect, there is provided a power system for anaircraft comprising:

-   a gas turbine engine arranged to burn a fuel so as to provide power    to the aircraft;-   one or more first fuel tanks arranged to be used to power    ground-based operation of the aircraft;-   one or more secondary fuel tanks, each arranged to contain a fuel to    be used to power the aircraft in flight; and-   a fuel manager arranged to control fuel supply so as to take fuel    from only the one or more first fuel tanks to power at least the    majority of ground-based operations.

Benefits may therefore be provided by filling the first fuel tank(s)with a fuel optimised for use in ground-based operations, e.g. for moreefficient running of the engine, and/or for reduced emissions. Havingone or more first fuel tanks arranged for, and optionally dedicated to,this purpose may facilitate refuelling and operation.

In some examples, only fuel from the first fuel tank(s) may be used topower aircraft operations on the ground, such that all ground-basedoperations are powered using fuel from the first fuel tank(s).

In other examples, most but not all of the fuel used for groundoperations is taken from the first fuel tank, with only small amountsfrom other sources being used (e.g. less than 10% or less than 5% of thefuel use and/or of the operation time of ground-based operations).

In many examples, a single first fuel tank is provided. However, it willbe appreciated that, whilst the discussion below often refers to asingle first fuel tank, the disclosure is not limited in that way.

In some examples, especially in examples where the fuel in the firstfuel tank has a higher viscosity at a given temperature than the fuel inanother fuel tank, fuel from another fuel tank may be used for start-upof the engine, and the fuel source may then be switched to the firstfuel tank once the engine is running, e.g. once a certain temperaturehas been reached. In such examples, fuel from the first fuel tank may beused for all ground-based operations except for engine start-up.

The fuel manager may be further arranged to take fuel only from the oneor more secondary fuel tanks for at least the majority of otheroperations (e.g. climb and cruise). It will be appreciated that anyremaining fuel in the first fuel tank may be finished off in flight(alone or as part of a blend); either in the early stages followingground-based operations, or thereafter.

The fuel manager may be arranged to take fuel from the one or moresecondary fuel tanks for at least the majority of other operations.

The fuel manager may be arranged to take fuel from only the first fueltank to power all ground-based operations.

A gas turbine engine of the one or more gas turbine engines may be a gasturbine engine of an Auxiliary Power Unit – APU. The APU may be arrangedto be active only during ground-based operations.

The first fuel tank may be arranged to provide fuel to the APU whenperforming operations on the ground, and to serve as a trim tank inflight.

The APU may not be arranged to provide any propulsive power to theaircraft.

A gas turbine engine of the one or more gas turbine engines may be a gasturbine engine arranged to provide propulsive power to the aircraft. Thegas turbine engine may comprise:

-   an engine core comprising a turbine, a compressor, and a core shaft    connecting the turbine to the compressor; and-   a fan located upstream of the engine core, the fan comprising a    plurality of fan blades and being arranged to be driven by an output    from the core shaft.

The fuel manager may be arranged to supply fuel only from the one ormore secondary fuel tanks to the gas turbine engine in flight, such thatthe first fuel tank is not used to supply fuel to an engine in flight.

According to a seventeenth aspect, there is provided a propulsion systemfor an aircraft comprising:

-   a gas turbine engine, the gas turbine engine optionally comprising:    -   an engine core comprising a turbine, a compressor, and a core        shaft connecting the turbine to the compressor; and    -   a fan located upstream of the engine core, the fan comprising a        plurality of fan blades and being arranged to be driven by an        output from the core shaft;-   one or more first fuel tanks arranged to be used to power    ground-based operation of the aircraft;-   one or more secondary fuel tanks, each arranged to contain a fuel to    be used to power the gas turbine engine in flight; and-   a fuel manager arranged to control fuel input to the gas turbine    engine so as to take fuel from only the one or more first fuel tanks    to power at least the majority of ground-based operations.

The fuel manager may be further arranged to take fuel only from the oneor more secondary fuel tanks for at least the majority of otheroperations (e.g. climb and cruise).

The first fuel tank may be arranged to contain only a fuel which is asustainable aviation fuel.

In examples with only one first fuel tank, the first fuel tank may besmaller than the one or more secondary fuel tanks. In examples withmultiple first fuel tanks, the total volume of the first fuel tanks maybe less than the total volume of the secondary fuel tanks, andoptionally smaller than the volume of each secondary fuel tankindividually.

The propulsion system may comprise a plurality of secondary fuel tanks.The fuel manager may be arranged to be able to mix fuels from thesecondary fuel tanks to power the gas turbine engine in flight, but maynot be able to mix fuel from the first fuel tank with fuel from thesecondary fuel tanks.

The fuel in the first fuel tank may have a lower calorific value and/ormay generate lower levels of nvPM emissions than any fuel stored in theone or more secondary fuel tanks.

According to an eighteenth aspect, there is provided a method ofoperating an aircraft comprising:

-   a gas turbine engine arranged to burn a fuel so as to provide power    to the aircraft;-   one or more first fuel tanks arranged to be used to power    ground-based operation of the aircraft; and-   one or more secondary fuel tanks, each arranged to contain a fuel to    be used to power the aircraft in flight,-   the method comprising:    -   controlling fuel supply so as to take fuel from only the one or        more first fuel tanks when the aircraft to power at least the        majority of ground-based operations.

The method may further comprise taking fuel from only the one or moresecondary fuel tanks for at least the majority of other operations.

In some examples, only fuel from the one or more secondary fuel tanksmay be used for other operations, such that the first fuel tank isexclusively used for ground-based operations.

The gas turbine engine may be arranged to provide propulsive power tothe aircraft, and may comprise:

-   an engine core comprising a turbine, a compressor, and a core shaft    connecting the turbine to the compressor; and-   a fan located upstream of the engine core, the fan comprising a    plurality of fan blades and being arranged to be driven by an output    from the core shaft.

The gas turbine engine may be a gas turbine engine of an Auxiliary PowerUnit – APU – of the aircraft.

The first fuel tank may be a trim tank of the aircraft. The fuel in thefirst fuel tank, which may be a sustainable aviation fuel, may bearranged to be used up performing the operations on the ground such thatthe first fuel tank is at last substantially empty and available toreceive fuel pumped thereinto in flight.

According to a nineteenth aspect, there is provided a method ofmodifying an aircraft comprising one or more gas turbine engines and aplurality of fuel tanks, the method comprising:

-   providing one or more first fuel tanks which are fluidly isolated    from other (secondary) fuel tanks of the plurality of fuel tanks;    and-   providing a fuel manager arranged to control fuel input to the one    or more gas turbine engines so as to take fuel from only the one or    more first fuel tanks to power at least the majority of ground-based    operations.

The fuel manager may further be arranged to take fuel from only the oneor more secondary fuel tanks for at least the majority of otheroperations.

The first fuel tank(s) may be permanently fluidly isolated from otherfuel tanks, or may be reversibly isolatable from the other fuel tanks,e.g. by means of one or more pumps and/or valves.

In some examples, only fuel from the one or more secondary fuel tanksmay be used for other operations, such that the first fuel tank isexclusively used for ground-based operations.

According to a twentieth aspect, there is provided a power system for anaircraft comprising:

-   an Auxiliary Power Unit – APU – comprising a gas turbine engine    arranged to burn a fuel so as to provide power to the aircraft; and-   one or more first fuel tanks which are fluidly isolated from any    other fuel tanks of the power system;-   and wherein the one or more first fuel tanks are dedicated to the    APU, such that all fuel used by the APU is taken from the one or    more first fuel tanks (in normal operation).

It will be appreciated that an aircraft is generally arranged such thatthe APU can also be provided with fuel from one or more other tanks, forexample in case the APU needs to start-up and operate in an emergencyduring flight, e.g. for non-propulsive purposes such as poweringaircraft flight control surfaces after a main engine flame-out and/orproviding power for restarting the main engines.

According to a twenty-first aspect, there is provided a power system foran aircraft comprising:

-   a gas turbine engine arranged to burn a fuel in a combustor so as to    provide power to the aircraft;-   a plurality of fuel tanks arranged to contain a different fuel to be    used to provide power to the aircraft, wherein a first fuel tank of    the plurality of fuel tanks is arranged to contain a first fuel, and    a second tank of the plurality of fuel tanks is arranged to contain    a second fuel with a different composition from the first fuel; and-   a fuel manager arranged to store information on the fuel contained    in each fuel tank and to control fuel supply so as to take fuel from    the second tank for engine start-up, before switching to the first    fuel tank.

The second fuel may be selected for its improved start-up properties;for example having a lower viscosity at a given temperature than thefuel in the first fuel tank, so as to facilitate cold start of anengine.

The fuel selected for its improved start-up properties may have a lowerviscosity at a given temperature than the fuel in the first tank.

The second fuel may be fossil-derived / petroleum-based.

The fuel manager may be arranged to control the fuel supply so as toswitch from taking fuel from the second tank to taking fuel from thefirst fuel tank when at least one of the following conditions is met:

-   (i) the fuel reaches a temperature of 60° C. at the inlet to the    combustor;-   (ii) the gas turbine engine has been running for a period of 30    seconds; and-   (iii) the gas turbine engine has reached idle operation.

The first tank may be arranged to contain a sustainable aviation fuel.

The second tank may be arranged to contain a fossil-based hydrocarbonfuel.

The gas turbine engine may be a gas turbine engine of an Auxiliary PowerUnit - APU. The APU may be arranged to be active only duringground-based operations, at least in normal operation.

The first fuel tank may be exclusively dedicated to the APU such thatfuel from the first fuel tank is not arranged to be provided to anyother gas turbine engine of the aircraft.

The first fuel tank may be arranged to provide fuel to the APU whenperforming operations on the ground, and to serve as a trim tank inflight.

The APU may be arranged not to provide any propulsive power to theaircraft.

Alternatively, the gas turbine engine may be arranged to providepropulsive power to the aircraft.

The gas turbine engine may comprise an engine core comprising a turbine,a compressor, and a core shaft connecting the turbine to the compressor;and a fan located upstream of the engine core, the fan comprising aplurality of fan blades and being arranged to be driven by an outputfrom the core shaft.

The first fuel in the first fuel tank may be selected such that the gasturbine engine can be run on that fuel alone. The second fuel in thesecond fuel tank may be selected such that the gas turbine engine can berun on that fuel alone in flight, as well as for start-up.

Each fuel tank may be arranged to contain a fuel with a different typeor proportion of a sustainable aviation fuel (SAF).

The fuel manager may be arranged to control fuel input to the gasturbine engine in flight by selection of a specific fuel or fuelcombination from one or more of the plurality of fuel tanks.

The first fuel may be SAF or a high %SAF blend, and the fuel manager maybe arranged to control the fuel supply so as to take fuel from the firstfuel tank for the majority of ground-based operations -start-up may bethe only exception.

According to a twenty-second aspect, there is provided a method ofoperating an aircraft comprising a gas turbine engine and a plurality offuel tanks arranged to store fuel to power the gas turbine engine, themethod comprising:

-   arranging at least two of the fuel tanks to store a different fuel,    wherein a first fuel tank of the plurality of fuel tanks is arranged    to contain a first fuel, and a second tank of the plurality of fuel    tanks is arranged to contain a second fuel with a different    composition from the first fuel; and-   controlling fuel supply so as to take fuel from the second tank for    engine start-up, before switching to the first fuel tank.

The method may further comprise storing information on the fuelcontained in each fuel tank. The control may be performed based on thestored information.

The gas turbine engine may be arranged to provide propulsive power tothe aircraft. The gas turbine engine may comprise an engine corecomprising a turbine, a compressor, and a core shaft connecting theturbine to the compressor; and a fan located upstream of the enginecore, the fan comprising a plurality of fan blades and being arranged tobe driven by an output from the core shaft.

The first fuel tank may be a trim tank of the aircraft. The first fuelmay be a sustainable aviation fuel (SAF) or a high %SAF blend and thefuel in the first fuel tank may be arranged to be used up performingoperations on the ground (after start-up) such that the first fuel tankis at last substantially empty by the end of climb, if not on take-off,and available to receive fuel pumped thereinto in flight.

According to a twenty-third aspect, there is provided a method ofmodifying an aircraft comprising a gas turbine engine and a plurality offuel tanks arranged to store fuel to power the gas turbine engine, themethod comprising:

-   arranging a first fuel tank of the plurality of fuel tanks to    contain a first fuel, and a second tank of the plurality of fuel    tanks to contain a second fuel with a different composition from the    first fuel; and-   providing a fuel manager arranged to control fuel supply so as to    take fuel from the second tank for engine start-up, before switching    to the first fuel tank.

The fuel manager may be additionally arranged to store information onthe fuel contained in each fuel tank. The control may be performed basedon the stored information.

The gas turbine engine may be arranged to provide propulsive power tothe aircraft, and may comprise an engine core comprising a turbine, acompressor, and a core shaft connecting the turbine to the compressor;and a fan located upstream of the engine core, the fan comprising aplurality of fan blades and being arranged to be driven by an outputfrom the core shaft.

The gas turbine engine may be a gas turbine engine of an Auxiliary PowerUnit – APU – of the aircraft.

According to a twenty-fourth aspect, there is provided a method ofoperating an aircraft comprising a gas turbine engine and a plurality offuel tanks arranged to provide fuel to the gas turbine engine, whereinat least two of the fuel tanks contain fuels with different fuelcharacteristics, the method being performed by processing circuitry andcomprising:

-   obtaining a flight profile for a flight of the aircraft; and-   determining a fuelling schedule for the flight based on the flight    profile and the fuel characteristics, the fuelling schedule    governing/dictating the variation with time of how much fuel is    drawn from each tank.

The fuelling schedule lists an intended variation with time of how muchfuel is drawn from each tank and is intended to be used to instruct afuelling manager to supply fuel to the gas turbine engine accordingly.The fuelling schedule can therefore be described as governing, dictatingor directing fuel use for the flight (optionally for the aircraft inflight only, or also for ground-based operations).

The method may be performed on-wing, e.g. by a fuelling scheduledetermination module of the aircraft, which may form a part of anelectronic engine controller (EEC) of the aircraft. Alternatively, themethod may be performed off-wing, and the fuelling schedule provided tothe aircraft for implementation.

The fuel characteristics of the fuel comprise at least one of:

-   i. percentage of sustainable aviation fuel in the fuel;-   ii. aromatic hydrocarbon content of the fuel;-   iii. multi-aromatic hydrocarbon content of the fuel;-   iv. percentage of nitrogen-containing species in the fuel;-   v. presence or percentage of a tracer species or trace element in    the fuel;-   vi. hydrogen to carbon ratio of the fuel;-   vii. hydrocarbon distribution of the fuel;-   viii. level of non-volatile particulate matter emissions on    combustion;-   ix. naphthalene content of the fuel;-   x. sulphur content of the fuel;-   xi. cycloparaffin content of the fuel;-   xii. oxygen content of the fuel;-   xiii. thermal stability of the fuel;-   xiv. level of coking of the fuel;-   xv. an indication that the fuel is a fossil fuel; and-   xvi. at least one of density, viscosity, calorific value, and heat    capacity.

The fuelling schedule may be determined using information from theflight profile including at least one of:

-   (i) intended altitude; and-   (ii) intended route.

The method may further comprise receiving forecast weather conditionsfor an intended route of the aircraft defined in the flight profile, andthe received forecast weather conditions may be used to influence thefuelling schedule.

The determining the fuelling schedule may comprise determining how muchsustainable aviation fuel – SAF – is available to the aircraft, and/orwhich tanks contain SAF or a high %SAF blend, and preferentiallyscheduling the use of SAF (alone or as part of a blend) for ground-basedoperations of the aircraft.

The determining the fuelling schedule may comprise determining acalorific value of each fuel onboard the aircraft, and preferentiallyscheduling the use of a lower calorific value fuel for periods of lowerthrust demand.

The method may further comprise controlling fuel input to the gasturbine engine in operation in accordance with the fuelling schedule.

The obtaining and determining steps may be performed off-wing. Themethod may further comprise providing the fuelling schedule to theaircraft prior to the controlling step.

According to a twenty-fifth aspect, there is provided a propulsionsystem for an aircraft comprising:

-   a gas turbine engine, the gas turbine engine optionally comprising:    -   an engine core comprising a turbine, a compressor, and a core        shaft connecting the turbine to the compressor; and    -   a fan located upstream of the engine core, the fan comprising a        plurality of fan blades and being arranged to be driven by an        output from the core shaft;-   a plurality of fuel tanks arranged to contain fuel to power the gas    turbine engine, wherein at least two of the fuel tanks contain fuels    with different fuel characteristics; and    -   a fuelling schedule determination module arranged to:        -   obtain a flight profile for a flight of the aircraft; and        -   determine a fuelling schedule for the flight based on the            flight profile and the fuel characteristics, the fuelling            schedule governing the variation with time of how much fuel            is drawn from each tank during the flight.

The fuel characteristics of the fuel may comprise one or more of thefuel characteristics listed above for the twenty-fourth aspect.

The fuelling schedule determination module may be arranged to determinethe fuelling schedule using information from the flight profileincluding at least one of:

-   (i) intended altitude; and-   (ii) intended route.

The propulsion system may further comprise a receiver arranged toreceive data concerning forecast weather conditions for an intendedroute of the aircraft, the route being defined in the flight profile.The received forecast weather conditions may be used to influence thefuelling schedule.

The fuelling schedule determination module may be arranged to determinethe fuelling schedule based on determining how much sustainable aviationfuel – SAF – is available to the aircraft, and to preferentiallyschedule the use of SAF for ground-based operations of the aircraft.

The fuelling schedule determination module may be arranged to determinethe fuelling schedule based on determining a calorific value of eachfuel onboard the aircraft, and to preferentially schedule the use of alower calorific value fuel for periods of lower thrust demand.

The fuelling schedule determination module may be arranged to controlfuel input to the gas turbine engine in operation in accordance with thefuelling schedule.

According to a twenty-sixth aspect, there is provided a non-transitorycomputer readable medium having stored thereon instructions that, whenexecuted by a processor, cause the processor to:

determine a fuelling schedule for a flight of an aircraft, the aircraftcomprising a gas turbine engine and a plurality of fuel tanks arrangedto provide fuel to the gas turbine engine, wherein at least two of thefuel tanks contain fuels with different fuel characteristics. Thefuelling schedule is determined based on a flight profile for the flightof the aircraft and the fuel characteristics of the fuels available tothe aircraft. The fuelling schedule lists/directs the variation withtime of how much fuel is drawn from each tank over the course of theflight.

The instructions may be further arranged to cause the processor tocontrol fuel input to the gas turbine engine in operation in accordancewith the fuelling schedule. The processor may comprise or consist of anonboard fuelling schedule determination module, and may be or may be apart of an Electronic Engine Controller.

The instructions may be further arranged to cause the processor toprovide the fuelling schedule to the aircraft for implementation. Theprocessor may comprise or consist of an off-wing fuelling scheduledetermination module.

According to a twenty-seventh aspect, there is provided a power systemfor an aircraft comprising:

-   one or more gas turbine engines arranged to burn a fuel so as to    provide power to the aircraft;-   a plurality of fuel tanks each arranged to contain a fuel to be used    to provide power to the aircraft, wherein at least two tanks of the    plurality of fuel tanks contain different fuels, the different fuels    each having a different proportion of a sustainable aviation fuel;    and-   a fuel manager arranged to:    -   store information on the fuel contained in each fuel tank;    -   identify which tank contains the fuel with the highest        proportion of a sustainable aviation fuel; and    -   control fuel supply so as to take fuel only from the tank        containing the fuel with the highest proportion of a sustainable        aviation fuel to power at least the majority of operations on        the ground.

The different proportion of a sustainable aviation fuel (SAF) may befrom 0% SAF to 100% SAF. The fuel with the highest proportion of asustainable aviation fuel may be greater than 50% SAF.

A gas turbine engine of the one or more gas turbine engines may be a gasturbine engine of an Auxiliary Power Unit – APU.

A first fuel tank of the plurality of fuel tanks may be arranged tocontain the fuel with the highest proportion of a sustainable aviationfuel, and optionally the fuel may be a sustainable aviation fuel (i.e. afuel for which the proportion of SAF is 100%). The first fuel tank maybe exclusively dedicated to the APU such that the fuel from the firstfuel tank is not arranged to be provided to any other gas turbine engineof the aircraft.

If a plurality of fuel tanks contain a fuel having the same highestproportion of a sustainable aviation fuel, which tank to use may beselected based on comparing at least one of:

-   (i) levels of non-volatile particulate matter emissions on    combustion of the fuels; and-   (ii) hydrogen to carbon ratios of the fuels.

One or more other parameters relating to air quality may also becompared so as to select the fuel likely to provide the best air qualityoutcomes. Environmental factors (e.g. airport altitude and humidity) mayalso be considered in this assessment.

The power system may be a propulsion system of the aircraft, and the gasturbine engine (at least one gas turbine engine of the one or more gasturbine engines) may be arranged to provide propulsive power to theaircraft.

The fuel with the highest proportion of a sustainable aviation fuel maybe selected such that the propulsion system can be run on that fuelalone.

The fuel with the highest proportion of a sustainable aviation fuel –SAF – may contain more than 50% SAF, and optionally may contain at least55% SAF.

The fuel manager may be arranged to control fuel input to the gasturbine engine in flight by selection of a specific fuel or fuelcombination from one or more of the plurality of fuel tanks.

A first fuel tank of the plurality of fuel tanks may be arranged tocontain the fuel with the highest proportion of a sustainable aviationfuel, and may be smaller than the one or more other fuel tanks. Thefirst fuel tank may be arranged to be used exclusively for ground-basedoperations of the aircraft.

The fuel with the highest proportion of a sustainable aviation fuel, foruse in ground-based operations, may have a lower calorific value thanany fuel stored in another fuel tank of the plurality of fuel tanks.

The fuel with the highest proportion of a sustainable aviation fuel maybe used to power all ground-based operations of the aircraft.

A first fuel tank of the plurality of fuel tanks may be arranged tocontain the fuel with the highest proportion of a sustainable aviationfuel and may be arranged to provide fuel to the gas turbine engine whenperforming operations on the ground, and to serve as a trim tank inflight.

The fuel with the highest proportion of a sustainable aviation fuel –SAF – may be 100% SAF.

According to a twenty-eighth aspect, there is provided a power systemfor an aircraft comprising:

-   a gas turbine engine arranged to burn a fuel so as to provide power    to the aircraft;-   a plurality of fuel tanks each arranged to contain a fuel to be used    to provide power to the aircraft, wherein at least two tanks of the    plurality of fuel tanks contain different fuels, a first tank    containing a fuel which is more than 50% sustainable aviation fuel    and a second tank containing a fuel which is less than 50%    sustainable aviation fuel; and-   a fuel manager arranged to:    -   store information on the fuel contained in each fuel tank; and    -   control fuel supply so as to only use fuel which is more than        50% a sustainable aviation fuel to power at least the majority        of operations on the ground.

The first tank may contain a fuel which is a sustainable aviation fuel(i.e. 100% SAF).

According to a twenty-ninth aspect, there is provided a method ofoperating an aircraft comprising a gas turbine engine and a plurality offuel tanks arranged to store fuel to power the gas turbine engine, themethod comprising:

-   arranging two or more fuel tanks of the plurality of fuel tanks to    store different fuels, the different fuels each having a different    proportion of a sustainable aviation fuel;-   identifying which tank contains the fuel with the highest proportion    of a sustainable aviation fuel; and-   controlling fuel supply so as to take fuel only from the tank    containing the fuel with the highest proportion of a sustainable    aviation fuel when the aircraft is performing at least the majority    of operations on the ground.

The method may further comprise storing information on the fuelcontained in each fuel tank. The control may be performed based on thestored information.

The gas turbine engine may be arranged to provide propulsive power tothe aircraft.

The gas turbine engine may be a gas turbine engine of an Auxiliary PowerUnit – APU – of the aircraft.

A first fuel tank of the one or more tanks may be arranged to containthe fuel with the highest proportion of a sustainable aviation fuel.This first fuel tank may be arranged to function as a trim tank of theaircraft - the fuel in the first fuel tank may therefore be arranged tobe (at least substantially) used up performing the operations on theground such that the first fuel tank is at last substantially empty andavailable to receive fuel pumped thereinto in flight.

According to a thirtieth aspect, there is provided a method of modifyingan aircraft comprising a gas turbine engine and a plurality of fueltanks arranged to store fuel to power the gas turbine engine, the methodcomprising:

-   arranging two or more fuel tanks of the plurality of fuel tanks to    store different fuels, the different fuels each having a different    proportion of a sustainable aviation fuel; and-   providing a fuel manager arranged to:    -   identify which tank contains the fuel with the highest        proportion of a sustainable aviation fuel; and    -   control fuel supply so as to take fuel only from the tank        containing the fuel with the highest proportion of a sustainable        aviation fuel when the aircraft is performing at least the        majority of operations on the ground.

The fuel manager may be arranged to store information on the fuelcontained in each fuel tank. The fuel manager may be arranged to performthe tank identification and control of the fuel supply based on thestored information.

As noted elsewhere herein, the present disclosure may relate to a gasturbine engine. Such a gas turbine engine may comprise an engine corecomprising a turbine, a combustor, a compressor, and a core shaftconnecting the turbine to the compressor. Such a gas turbine engine maycomprise a fan (having fan blades) located upstream of the engine core.Alternatively, in some examples, the gas turbine engine may comprise afan located downstream of the engine core. Thus, the gas turbine enginemay be an open rotor or a turboprop engine.

Where the gas turbine engine is an open rotor or a turboprop engine, thegas turbine engine may comprise two contra-rotating propeller stagesattached to and driven by a free power turbine via a shaft. Thepropellers may rotate in opposite senses so that one rotates clockwiseand the other anticlockwise around the engine’s rotational axis.Alternatively, the gas turbine engine may comprise a propeller stage anda guide vane stage configured downstream of the propeller stage. Theguide vane stage may be of variable pitch. Accordingly, high-pressure,intermediate pressure, and free power turbines respectively may drivehigh and intermediate pressure compressors and propellers by suitableinterconnecting shafts. Thus, the propellers may provide the majority ofthe propulsive thrust.

Where the gas turbine engine is an open rotor or a turboprop engine, oneor more of the propellor stages may be driven by a gearbox of the typedescribed.

Arrangements of the present disclosure may be particularly, although notexclusively, beneficial for fans that are driven via a gearbox.Accordingly, the gas turbine engine may comprise a gearbox that receivesan input from the core shaft and outputs drive to the fan so as to drivethe fan at a lower rotational speed than the core shaft. The input tothe gearbox may be directly from the core shaft, or indirectly from thecore shaft, for example via a spur shaft and/or gear. The core shaft mayrigidly connect the turbine and the compressor, such that the turbineand compressor rotate at the same speed (with the fan rotating at alower speed).

The gas turbine engine as described and/or claimed herein may have anysuitable general architecture. For example, the gas turbine engine mayhave any desired number of shafts that connect turbines and compressors,for example one, two or three shafts. Purely by way of example, theturbine connected to the core shaft may be a first turbine, thecompressor connected to the core shaft may be a first compressor, andthe core shaft may be a first core shaft. The engine core may furthercomprise a second turbine, a second compressor, and a second core shaftconnecting the second turbine to the second compressor. The secondturbine, second compressor, and second core shaft may be arranged torotate at a higher rotational speed than the first core shaft.

In such an arrangement, the second compressor may be positioned axiallydownstream of the first compressor. The second compressor may bearranged to receive (for example directly receive, for example via agenerally annular duct) flow from the first compressor.

The gearbox may be arranged to be driven by the core shaft that isconfigured to rotate (for example in use) at the lowest rotational speed(for example the first core shaft in the example above). For example,the gearbox may be arranged to be driven only by the core shaft that isconfigured to rotate (for example in use) at the lowest rotational speed(for example only be the first core shaft, and not the second coreshaft, in the example above). Alternatively, the gearbox may be arrangedto be driven by any one or more shafts, for example the first and/orsecond shafts in the example above.

The gearbox may be a reduction gearbox (in that the output to the fan isa lower rotational rate than the input from the core shaft). Any type ofgearbox may be used. For example, the gearbox may be a “planetary” or“star” gearbox, as described in more detail elsewhere herein. Thegearbox may have any desired reduction ratio (defined as the rotationalspeed of the input shaft divided by the rotational speed of the outputshaft), for example greater than 2.5, for example in the range of from 3to 4.2, or 3.2 to 3.8, for example on the order of or at least 3, 3.1,3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1 or 4.2. The gear ratiomay be, for example, between any two of the values in the previoussentence. Purely by way of example, the gearbox may be a “star” gearboxhaving a ratio in the range of from 3.1 or 3.2 to 3.8. In somearrangements, the gear ratio may be outside these ranges.

In any gas turbine engine as described and/or claimed herein, fuel of agiven composition or blend is provided to a combustor, which may beprovided axially downstream of the fan and compressor(s). For example,the combustor may be directly downstream of (for example at the exit of)the second compressor, where a second compressor is provided. By way offurther example, the flow at the exit to the combustor may be providedto the inlet of the second turbine, where a second turbine is provided.The combustor may be provided upstream of the turbine(s).

The or each compressor (for example the first compressor and secondcompressor as described above) may comprise any number of stages, forexample multiple stages. Each stage may comprise a row of rotor bladesand a row of stator vanes, which may be variable stator vanes (in thattheir angle of incidence may be variable). The row of rotor blades andthe row of stator vanes may be axially offset from each other.

The or each turbine (for example the first turbine and second turbine asdescribed above) may comprise any number of stages, for example multiplestages. Each stage may comprise a row of rotor blades and a row ofstator vanes. The row of rotor blades and the row of stator vanes may beaxially offset from each other.

Each fan blade may be defined as having a radial span extending from aroot (or hub) at a radially inner gas-washed location, or 0% spanposition, to a tip at a 100% span position. The ratio of the radius ofthe fan blade at the hub to the radius of the fan blade at the tip maybe less than (or on the order of) any of: 0.4, 0.39, 0.38, 0.37, 0.36,0.35, 0.34, 0.33, 0.32, 0.31, 0.3, 0.29, 0.28, 0.27, 0.26, or 0.25. Theratio of the radius of the fan blade at the hub to the radius of the fanblade at the tip may be in an inclusive range bounded by any two of thevalues in the previous sentence (i.e. the values may form upper or lowerbounds), for example in the range of from 0.28 to 0.32. These ratios maycommonly be referred to as the hub-to-tip ratio. The radius at the huband the radius at the tip may both be measured at the leading edge (oraxially forwardmost) part of the blade. The hub-to-tip ratio refers, ofcourse, to the gas-washed portion of the fan blade, i.e. the portionradially outside any platform.

The radius of the fan may be measured between the engine centreline andthe tip of a fan blade at its leading edge. The fan diameter (which maysimply be twice the radius of the fan) may be greater than (or on theorder of) any of: 220 cm, 230 cm, 240 cm, 250 cm (around 100 inches),260 cm, 270 cm (around 105 inches), 280 cm (around 110 inches), 290 cm(around 115 inches), 300 cm (around 120 inches), 310 cm, 320 cm (around125 inches), 330 cm (around 130 inches), 340 cm (around 135 inches), 350cm, 360 cm (around 140 inches), 370 cm (around 145 inches), 380 (around150 inches) cm, 390 cm (around 155 inches), 400 cm, 410 cm (around 160inches) or 420 cm (around 165 inches). The fan diameter may be in aninclusive range bounded by any two of the values in the previoussentence (i.e. the values may form upper or lower bounds), for examplein the range of from 240 cm to 280 cm or 330 cm to 380 cm.

The rotational speed of the fan may vary in use. Generally, therotational speed is lower for fans with a higher diameter. Purely by wayof non-limitative example, the rotational speed of the fan at cruiseconditions may be less than 2500 rpm, for example less than 2300 rpm.Purely by way of further non-limitative example, the rotational speed ofthe fan at cruise conditions for an engine having a fan diameter in therange of from 220 cm to 300 cm (for example 240 cm to 280 cm or 250 cmto 270 cm) may be in the range of from 1700 rpm to 2500 rpm, for examplein the range of from 1800 rpm to 2300 rpm, for example in the range offrom 1900 rpm to 2100 rpm. Purely by way of further non-limitativeexample, the rotational speed of the fan at cruise conditions for anengine having a fan diameter in the range of from 330 cm to 380 cm maybe in the range of from 1200 rpm to 2000 rpm, for example in the rangeof from 1300 rpm to 1800 rpm, for example in the range of from 1400 rpmto 1800 rpm.

In use of the gas turbine engine, the fan (with associated fan blades)rotates about a rotational axis. This rotation results in the tip of thefan blade moving with a velocity U_(tip). The work done by the fanblades 13 on the flow results in an enthalpy rise dH of the flow. A fantip loading may be defined as dH/U_(tip) ², where dH is the enthalpyrise (for example the 1-D average enthalpy rise) across the fan andU_(tip) is the (translational) velocity of the fan tip, for example atthe leading edge of the tip (which may be defined as fan tip radius atleading edge multiplied by angular speed). The fan tip loading at cruiseconditions may be greater than (or on the order of) any of: 0.28, 0.29,0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39 or 0.4 (allvalues being dimensionless). The fan tip loading may be in an inclusiverange bounded by any two of the values in the previous sentence (i.e.the values may form upper or lower bounds), for example in the range offrom 0.28 to 0.31, or 0.29 to 0.3.

Gas turbine engines in accordance with the present disclosure may haveany desired bypass ratio, where the bypass ratio is defined as the ratioof the mass flow rate of the flow through the bypass duct to the massflow rate of the flow through the core at cruise conditions. In somearrangements the bypass ratio may be greater than (or on the order of)any of the following: 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5,15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5 or 20. The bypass ratiomay be in an inclusive range bounded by any two of the values in theprevious sentence (i.e. the values may form upper or lower bounds), forexample in the range of form 12 to 16, 13 to 15, or 13 to 14. The bypassduct may be substantially annular. The bypass duct may be radiallyoutside the core engine. The radially outer surface of the bypass ductmay be defined by a nacelle and/or a fan case.

The overall pressure ratio of a gas turbine engine as described and/orclaimed herein may be defined as the ratio of the stagnation pressureupstream of the fan to the stagnation pressure at the exit of thehighest pressure compressor (before entry into the combustor). By way ofnon-limitative example, the overall pressure ratio of a gas turbineengine as described and/or claimed herein at cruise may be greater than(or on the order of) any of the following: 35, 40, 45, 50, 55, 60, 65,70, 75. The overall pressure ratio may be in an inclusive range boundedby any two of the values in the previous sentence (i.e. the values mayform upper or lower bounds), for example in the range of from 50 to 70.

Specific thrust of an engine may be defined as the net thrust of theengine divided by the total mass flow through the engine. In someexamples, specific thrust may depend, for a given thrust condition, uponthe specific composition of fuel provided to the combustor. At cruiseconditions, the specific thrust of an engine described and/or claimedherein may be less than (or on the order of) any of the following: 110Nkg⁻¹s, 105 Nkg⁻¹s, 100 Nkg⁻¹s, 95 Nkg⁻¹s, 90 Nkg⁻¹s, 85 Nkg⁻¹s or 80Nkg⁻¹s. The specific thrust may be in an inclusive range bounded by anytwo of the values in the previous sentence (i.e. the values may formupper or lower bounds), for example in the range of from 80 Nkg⁻¹s to100 Nkg⁻¹s, or 85 Nkg⁻¹s to 95 Nkg⁻¹s. Such engines may be particularlyefficient in comparison with conventional gas turbine engines.

A gas turbine engine as described and/or claimed herein may have anydesired maximum thrust. Purely by way of non-limitative example, a gasturbine as described and/or claimed herein may be capable of producing amaximum thrust of at least (or on the order of) any of the following:160 kN, 170 kN, 180 kN, 190 kN, 200 kN, 250 kN, 300 kN, 350 kN, 400 kN,450 kN, 500 kN, or 550 kN. The maximum thrust may be in an inclusiverange bounded by any two of the values in the previous sentence (i.e.the values may form upper or lower bounds). Purely by way of example, agas turbine as described and/or claimed herein may be capable ofproducing a maximum thrust in the range of from 330 kN to 420 kN, forexample 350 kN to 400 kN. The thrust referred to above may be themaximum net thrust at standard atmospheric conditions at sea level plus15° C. (ambient pressure 101.3 kPa, temperature 30° C.), with the enginestatic.

In use, the temperature of the flow at the entry to the high pressureturbine may be particularly high. This temperature, which may bereferred to as TET, may be measured at the exit to the combustor, forexample immediately upstream of the first turbine vane, which itself maybe referred to as a nozzle guide vane. In some examples, TET may depend,for a given thrust condition, upon the specific composition of fuelprovided to the combustor. At cruise, the TET may be at least (or on theorder of) any of the following: 1400 K, 1450 K, 1500 K, 1550 K, 1600 Kor 1650 K. The TET at cruise may be in an inclusive range bounded by anytwo of the values in the previous sentence (i.e. the values may formupper or lower bounds). The maximum TET in use of the engine may be, forexample, at least (or on the order of) any of the following: 1700 K,1750 K, 1800 K, 1850 K, 1900 K, 1950 K or 2000 K. The maximum TET may bein an inclusive range bounded by any two of the values in the previoussentence (i.e. the values may form upper or lower bounds), for examplein the range of from 1800 K to 1950 K. The maximum TET may occur, forexample, at a high thrust condition, for example at a maximum take-off(MTO) condition.

A fan blade and/or aerofoil portion of a fan blade described and/orclaimed herein may be manufactured from any suitable material orcombination of materials. For example at least a part of the fan bladeand/or aerofoil may be manufactured at least in part from a composite,for example a metal matrix composite and/or an organic matrix composite,such as carbon fibre. By way of further example at least a part of thefan blade and/or aerofoil may be manufactured at least in part from ametal, such as a titanium based metal or an aluminium based material(such as an aluminium-lithium alloy) or a steel based material. The fanblade may comprise at least two regions manufactured using differentmaterials. For example, the fan blade may have a protective leadingedge, which may be manufactured using a material that is better able toresist impact (for example from birds, ice or other material) than therest of the blade. Such a leading edge may, for example, be manufacturedusing titanium or a titanium-based alloy. Thus, purely by way ofexample, the fan blade may have a carbon-fibre or aluminium based body(such as an aluminium lithium alloy) with a titanium leading edge.

A fan as described and/or claimed herein may comprise a central portion,from which the fan blades may extend, for example in a radial direction.The fan blades may be attached to the central portion in any desiredmanner. For example, each fan blade may comprise a fixture which mayengage a corresponding slot in the hub (or disc). Purely by way ofexample, such a fixture may be in the form of a dovetail that may slotinto and/or engage a corresponding slot in the hub/disc in order to fixthe fan blade to the hub/disc. By way of further example, the fan bladesmaybe formed integrally with a central portion. Such an arrangement maybe referred to as a bladed disc or a bladed ring. Any suitable methodmay be used to manufacture such a bladed disc or bladed ring. Forexample, at least a part of the fan blades may be machined from a blockand/or at least part of the fan blades may be attached to the hub/discby welding, such as linear friction welding.

The gas turbine engines described and/or claimed herein may or may notbe provided with a variable area nozzle (VAN). Such a variable areanozzle may allow the exit area of the bypass duct to be varied in use.The general principles of the present disclosure may apply to engineswith or without a VAN.

The fan of a gas turbine as described and/or claimed herein may have anydesired number of fan blades, for example 14, 16, 18, 20, 22, 24 or 26fan blades.

As used herein, the terms idle, taxi, take-off, climb, cruise, descent,approach, and landing have the conventional meaning and would be readilyunderstood by the skilled person. Thus, for a given gas turbine enginefor an aircraft, the skilled person would immediately recognise eachterm to refer to an operating phase of the engine within a given missionof an aircraft to which the gas turbine engine is designed to beattached.

In this regard, ground idle may refer to an operating phase of theengine where the aircraft is stationary and in contact with the ground,but where there is a requirement for the engine to be running. Duringidle, the engine may be producing between 3% and 9% of the availablethrust of the engine. In further examples, the engine may be producingbetween 5% and 8% of available thrust. In yet further examples, theengine may be producing between 6% and 7% of available thrust. Taxi mayrefer to an operating phase of the engine where the aircraft is beingpropelled along the ground by the thrust produced by the engine. Duringtaxi, the engine may be producing between 5% and 15% of availablethrust. In further examples, the engine may be producing between 6% and12% of available thrust. In yet further examples, the engine may beproducing between 7% and 10% of available thrust. Take-off may refer toan operating phase of the engine where the aircraft is being propelledby the thrust produced by the engine. At an initial stage within thetake-off phase, the aircraft may be propelled whilst the aircraft is incontact with the ground. At a later stage within the take-off phase, theaircraft may be propelled whilst the aircraft is not in contact with theground. During take-off, the engine may be producing between 90% and100% of available thrust. In further examples, the engine may beproducing between 95% and 100% of available thrust. In yet furtherexamples, the engine may be producing 100% of available thrust.

Climb may refer to an operating phase of the engine where the aircraftis being propelled by the thrust produced by the engine. During climb,the engine may be producing between 75% and 100% of available thrust. Infurther examples, the engine may be producing between 80% and 95% ofavailable thrust. In yet further examples, the engine may be producingbetween 85% and 90% of available thrust. In this regard, climb may referto an operating phase within an aircraft flight cycle between take-offand the arrival at cruise conditions. Additionally or alternatively,climb may refer to a nominal point in an aircraft flight cycle betweentake-off and landing, where a relative increase in altitude is required,which may require an additional thrust demand of the engine.

As used herein, cruise conditions have the conventional meaning andwould be readily understood by the skilled person. Thus, for a given gasturbine engine for an aircraft, the skilled person would immediatelyrecognise cruise conditions to mean the operating point of the engine atmid-cruise of a given mission (which may be referred to in the industryas the “economic mission”) of an aircraft to which the gas turbineengine is designed to be attached. In this regard, mid-cruise is thepoint in an aircraft flight cycle at which 50% of the total fuel that isburned between top of climb and start of descent has been burned (whichmay be approximated by the midpoint – in terms of time and/or distance –between top of climb and start of descent. Cruise conditions thus definean operating point of, the gas turbine engine that provides a thrustthat would ensure steady state operation (i.e. maintaining a constantaltitude and constant Mach Number) at mid-cruise of an aircraft to whichit is designed to be attached, taking into account the number of enginesprovided to that aircraft. For example where an engine is designed to beattached to an aircraft that has two engines of the same type, at cruiseconditions the engine provides half of the total thrust that would berequired for steady state operation of that aircraft at mid-cruise.

In other words, for a given gas turbine engine for an aircraft, cruiseconditions are defined as the operating point of the engine thatprovides a specified thrust (required to provide – in combination withany other engines on the aircraft – steady state operation of theaircraft to which it is designed to be attached at a given mid-cruiseMach Number) at the mid-cruise atmospheric conditions (defined by theInternational Standard Atmosphere according to ISO 2533 at themid-cruise altitude). For any given gas turbine engine for an aircraft,the mid-cruise thrust, atmospheric conditions and Mach Number are known,and thus the operating point of the engine at cruise conditions isclearly defined.

Purely by way of example, the forward speed at the cruise condition maybe any point in the range of from Mach 0.7 to 0.9, for example 0.75 to0.85, for example 0.76 to 0.84, for example 0.77 to 0.83, for example0.78 to 0.82, for example 0.79 to 0.81, for example on the order of Mach0.8, on the order of Mach 0.85 or in the range of from 0.8 to 0.85. Anysingle speed within these ranges may be part of the cruise condition.For some aircraft, the cruise conditions may be outside these ranges,for example below Mach 0.7 or above Mach 0.9.

Purely by way of example, the cruise conditions may correspond tostandard atmospheric conditions (according to the International StandardAtmosphere, ISA) at an altitude that is in the range of from 10000 m to15000 m, for example in the range of from 10000 m to 12000 m, forexample in the range of from 10400 m to 11600 m (around 38000 ft), forexample in the range of from 10500 m to 11500 m, for example in therange of from 10600 m to 11400 m, for example in the range of from 10700m (around 35000 ft) to 11300 m, for example in the range of from 10800 mto 11200 m, for example in the range of from 10900 m to 11100 m, forexample on the order of 11000 m. The cruise conditions may correspond tostandard atmospheric conditions at any given altitude in these ranges.

Purely by way of example, the cruise conditions may correspond to anoperating point of the engine that provides a known required thrustlevel (for example a value in the range of from 30 kN to 35 kN) at aforward Mach number of 0.8 and standard atmospheric conditions(according to the International Standard Atmosphere) at an altitude of38000 ft (11582 m). Purely by way of further example, the cruiseconditions may correspond to an operating point of the engine thatprovides a known required thrust level (for example a value in the rangeof from 50 kN to 65 kN) at a forward Mach number of 0.85 and standardatmospheric conditions (according to the International StandardAtmosphere) at an altitude of 35000 ft (10668 m).

In use, a gas turbine engine described and/or claimed herein may operateat the cruise conditions defined elsewhere herein. Such cruiseconditions may be determined by the cruise conditions (for example themid-cruise conditions) of an aircraft to which at least one (for example2 or 4) gas turbine engine may be mounted in order to provide propulsivethrust.

Furthermore, the skilled person would immediately recognise either orboth of descent and approach to refer to an operating phase within anaircraft flight cycle between cruise and landing of the aircraft. Duringeither or both of descent and approach, the engine may be producingbetween 20% and 50% of available thrust. In further examples, the enginemay be producing between 25% and 40% of available thrust. In yet furtherexamples, the engine may be producing between 30% and 35% of availablethrust. Additionally or alternatively, descent may refer to a nominalpoint in an aircraft flight cycle between take-off and landing, where arelative decrease in altitude is required, and which may require areduced thrust demand of the engine.

According to an aspect, there is provided an aircraft comprising a gasturbine engine as described and/or claimed herein. The aircraftaccording to this aspect is the aircraft for which the gas turbineengine has been designed to be attached. Accordingly, the cruiseconditions according to this aspect correspond to the mid-cruise of theaircraft, as defined elsewhere herein.

According to an aspect, there is provided a method of operating a gasturbine engine as described and/or claimed herein. The operation may beat the cruise conditions as defined elsewhere herein (for example interms of the thrust, atmospheric conditions and Mach Number).

According to an aspect, there is provided a method of operating anaircraft comprising a gas turbine engine as described and/or claimedherein. The operation according to this aspect may include (or may be)operation at the mid-cruise of the aircraft, as defined elsewhereherein.

The skilled person will appreciate that except where mutually exclusive,a feature or parameter described in relation to any one of the aboveaspects may be applied to any other aspect. Furthermore, except wheremutually exclusive, any feature or parameter described herein may beapplied to any aspect and/or combined with any other feature orparameter described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with referenceto the Figures, in which:

FIG. 1 is a sectional side view of a gas turbine engine;

FIG. 2 is a close up sectional side view of an upstream portion of a gasturbine engine;

FIG. 3 is a partially cut-away view of a gearbox for a gas turbineengine;

FIG. 4 is a schematic view of an aircraft including a fuel compositiontracker;

FIG. 5 is a schematic representation of a fuel identification method;

FIG. 6 is a schematic view of an aircraft fuel composition trackingsystem, in context with a fuel supply line and on-board tank, indicatingoptional use as a fuel composition determination module;

FIG. 7 is a schematic view of an aircraft including a fuel compositiondetermination module;

FIG. 8 is a schematic representation of an aircraft operation method;

FIG. 9 is a schematic representation of another aircraft operationmethod;

FIG. 10 is a schematic view of an aircraft including a fuel manager;

FIG. 11 is a schematic representation of another aircraft operationmethod;

FIG. 12 is a schematic view of an aircraft fuel delivery system, incontext with a fuel tank and gas turbine engine;

FIG. 13 is a schematic representation of an aircraft modificationmethod; and

FIG. 14 is a schematic view of an aircraft with a different tankarrangement from that shown in FIG. 4 , FIG. 7 , or FIG. 10 , includinga fuel manager and a trim tank;

FIG. 15 is a schematic representation of another aircraft operationmethod;

FIG. 16 is a schematic view of an aircraft fuel delivery system, incontext with a fuel tank, an APU, and a gas turbine engine;

FIG. 17 is a schematic representation of another aircraft modificationmethod; and

FIG. 18 is a schematic view of an aircraft with a different tankarrangement from that shown in FIG. 14 ;

FIG. 19 is a schematic view of an aircraft including a fuel manager andhaving a different tank arrangement from that shown in FIG. 14 ;

FIG. 20 is a schematic representation of an aircraft operation method;

FIG. 21 is a schematic view of an aircraft fuel delivery system, incontext with fuel tanks and a gas turbine engine;

FIG. 22 is a schematic representation of an aircraft modificationmethod;

FIG. 23 is a schematic representation of an aircraft operation method;

FIG. 24 is a schematic representation of an aircraft modificationmethod; and

FIG. 25 is a schematic representation of an aircraft operation method.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1 illustrates a gas turbine engine 10 having a principal rotationalaxis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23that generates two airflows: a core airflow A and a bypass airflow B.The gas turbine engine 10 comprises a core 11 that receives the coreairflow A. The engine core 11 comprises, in axial flow series, a lowpressure compressor 14, a high-pressure compressor 15, combustionequipment 16, a high-pressure turbine 17, a low pressure turbine 19 anda core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. Thebypass airflow B flows through the bypass duct 22. The fan 23 isattached to and driven by the low pressure turbine 19 via a shaft 26 andan epicyclic gearbox 30.

In use, the core airflow A is accelerated and compressed by the lowpressure compressor 14 and directed into the high pressure compressor 15where further compression takes place. The compressed air exhausted fromthe high pressure compressor 15 is directed into the combustionequipment 16 where it is mixed with fuel F and the mixture is combusted.The resultant hot combustion products then expand through, and therebydrive, the high pressure and low pressure turbines 17, 19 before beingexhausted through the nozzle 20 to provide some propulsive thrust. Thehigh pressure turbine 17 drives the high pressure compressor 15 by asuitable interconnecting shaft 27. The fan 23 generally provides themajority of the propulsive thrust. The epicyclic gearbox 30 is areduction gearbox.

An exemplary arrangement for a geared fan gas turbine engine 10 is shownin FIG. 2 . The low pressure turbine 19 (see FIG. 1 ) drives the shaft26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclicgear arrangement 30. Radially outwardly of the sun gear 28 andintermeshing therewith is a plurality of planet gears 32 that arecoupled together by a planet carrier 34. The planet carrier 34constrains the planet gears 32 to precess around the sun gear 28 insynchronicity whilst enabling each planet gear 32 to rotate about itsown axis. The planet carrier 34 is coupled via linkages 36 to the fan 23in order to drive its rotation about the engine axis 9. Radiallyoutwardly of the planet gears 32 and intermeshing therewith is anannulus or ring gear 38 that is coupled, via linkages 40, to astationary supporting structure 24.

Note that the terms “low pressure turbine” and “low pressure compressor”as used herein may be taken to mean the lowest pressure turbine stagesand lowest pressure compressor stages (i.e. not including the fan 23)respectively and/or the turbine and compressor stages that are connectedtogether by the interconnecting shaft 26 with the lowest rotationalspeed in the engine (i.e. not including the gearbox output shaft thatdrives the fan 23). In some literature, the “low pressure turbine” and“low pressure compressor” referred to herein may alternatively be knownas the “intermediate pressure turbine” and “intermediate pressurecompressor”. Where such alternative nomenclature is used, the fan 23 maybe referred to as a first, or lowest pressure, compression stage.

The epicyclic gearbox 30 is shown by way of example in greater detail inFIG. 3 . Each of the sun gear 28, planet gears 32 and ring gear 38comprise teeth about their periphery to intermesh with the other gears.However, for clarity only exemplary portions of the teeth areillustrated in FIG. 3 . There are four planet gears 32 illustrated,although it will be apparent to the skilled reader that more or fewerplanet gears 32 may be provided within the scope of the claimedinvention. Practical applications of a planetary epicyclic gearbox 30generally comprise at least three planet gears 32.

The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3is of the planetary type, in that the planet carrier 34 is coupled to anoutput shaft via linkages 36, with the ring gear 38 fixed. However, anyother suitable type of epicyclic gearbox 30 may be used. By way offurther example, the epicyclic gearbox 30 may be a star arrangement, inwhich the planet carrier 34 is held fixed, with the ring (or annulus)gear 38 allowed to rotate. In such an arrangement the fan 23 is drivenby the ring gear 38. By way of further alternative example, the gearbox30 may be a differential gearbox in which the ring gear 38 and theplanet carrier 34 are both allowed to rotate.

It will be appreciated that the arrangement shown in FIGS. 2 and 3 is byway of example only, and various alternatives are within the scope ofthe present disclosure. Purely by way of example, any suitablearrangement may be used for locating the gearbox 30 in the engine 10and/or for connecting the gearbox 30 to the engine 10. By way of furtherexample, the connections (such as the linkages 36, 40 in the FIG. 2example) between the gearbox 30 and other parts of the engine 10 (suchas the input shaft 26, the output shaft and the fixed structure 24) mayhave any desired degree of stiffness or flexibility. By way of furtherexample, any suitable arrangement of the bearings between rotating andstationary parts of the engine (for example between the input and outputshafts from the gearbox and the fixed structures, such as the gearboxcasing) may be used, and the disclosure is not limited to the exemplaryarrangement of FIG. 2 . For example, where the gearbox 30 has a stararrangement (described above), the skilled person would readilyunderstand that the arrangement of output and support linkages andbearing locations would typically be different to that shown by way ofexample in FIG. 2 .

Accordingly, the present disclosure extends to a gas turbine enginehaving any arrangement of gearbox styles (for example star orplanetary), support structures, input and output shaft arrangement, andbearing locations.

Optionally, the gearbox may drive additional and/or alternativecomponents (e.g. the intermediate pressure compressor and/or a boostercompressor).

Other gas turbine engines to which the present disclosure may be appliedmay have alternative configurations. For example, such engines may havean alternative number of compressors and/or turbines and/or analternative number of interconnecting shafts. By way of further example,the gas turbine engine shown in FIG. 1 has a split flow nozzle 18, 20meaning that the flow through the bypass duct 22 has its own nozzle 18that is separate to and radially outside the core engine nozzle 20.However, this is not limiting, and any aspect of the present disclosuremay also apply to engines in which the flow through the bypass duct 22and the flow through the core 11 are mixed, or combined, before (orupstream of) a single nozzle, which may be referred to as a mixed flownozzle. One or both nozzles (whether mixed or split flow) may have afixed or variable area.

Whilst the described example relates to a turbofan engine, thedisclosure may apply, for example, to any type of gas turbine engine,such as an open rotor (in which the fan stage is not surrounded by anacelle) or turboprop engine, for example. In some arrangements, the gasturbine engine 10 may not comprise a gearbox 30.

The geometry of the gas turbine engine 10, and components thereof, isdefined by a conventional axis system, comprising an axial direction(which is aligned with the rotational axis 9), a radial direction (inthe bottom-to-top direction in FIG. 1 ), and a circumferential direction(perpendicular to the page in the FIG. 1 view). The axial, radial andcircumferential directions are mutually perpendicular.

The fuel F provided to the combustion equipment 16 may comprise afossil-based hydrocarbon fuel, such as Kerosene. Thus, the fuel F maycomprise molecules from one or more of the chemical families ofn-alkanes, iso-alkanes, cycloalkanes, and aromatics. Additionally oralternatively, the fuel F may comprise renewable hydrocarbons producedfrom biological or non-biological resources, otherwise known assustainable aviation fuel (SAF). In each of the provided examples, thefuel F may comprise one or more trace elements including, for example,sulphur, nitrogen, oxygen, inorganics, and metals.

Functional performance of a given composition, or blend of fuel for usein a given mission, may be defined, at least in part, by the ability ofthe fuel to service the Brayton cycle of the gas turbine engine 10.Parameters defining functional performance may include, for example,specific energy; energy density; thermal stability; and, emissionsincluding particulate matter. A relatively higher specific energy (i.e.energy per unit mass), expressed as MJ/kg, may at least partially reducetake-off weight, thus potentially providing a relative improvement infuel efficiency. A relatively higher energy density (i.e. energy perunit volume), expressed as MJ/L, may at least partially reduce take-offfuel volume, which may be particularly important for volume-limitedmissions or military operations involving refuelling. A relativelyhigher thermal stability (i.e. inhibition of fuel to degrade or cokeunder thermal stress) may permit the fuel to sustain elevatedtemperatures in the engine and fuel injectors, thus potentiallyproviding relative improvements in combustion efficiency. Reducedemissions, including particulate matter, may permit reduced contrailformation, whilst reducing the environmental impact of a given mission.Other properties of the fuel may also be key to functional performance.For example, a relatively lower freeze point (°C) may allow long-rangemissions to optimise flight profiles; minimum aromatic concentrations(%) may ensure sufficient swelling of certain materials used in theconstruction of o-rings and seals that have been previously exposed tofuels with high aromatic contents; and, a maximum surface tension (mN/m)may ensure sufficient spray break-up and atomisation of the fuel.

The ratio of the number of hydrogen atoms to the number of carbon atomsin a molecule may influence the specific energy of a given composition,or blend of fuel. Fuels with higher ratios of hydrogen atoms to carbonatoms may have higher specific energies in the absence of bond strain.For example, fossil-based hydrocarbon fuels may comprise molecules withapproximately 7 to 18 carbons, with a significant portion of a givencomposition stemming from molecules with 9 to 15 carbons, with anaverage of 12 carbons.

ASTM International (ASTM) D7566, Standard Specification for AviationTurbine Fuels Containing Synthesized Hydrocarbons (ASTM 2019c) approvesa number of sustainable aviation fuel blends comprising between 10% and50% sustainable aviation fuel (the remainder comprising one or morefossil-based hydrocarbon fuels, such as Kerosene), with furthercompositions awaiting approval. However, there is an anticipation in theaviation industry that sustainable aviation fuel blends comprising up to(and including) 100% sustainable aviation fuel (SAF) will be eventuallyapproved for use.

Sustainable aviation fuels may comprise one or more of n-alkanes,iso-alkanes, cyclo-alkanes, and aromatics, and may be produced, forexample, from one or more of synthesis gas (syngas); lipids (e.g. fats,oils, and greases); sugars; and alcohols. Thus, sustainable aviationfuels may comprise either or both of a lower aromatic and sulphurcontent, relative to fossil-based hydrocarbon fuels. Additionally oralternatively, sustainable aviation fuels may comprise either or both ofa higher iso-alkane and cyclo-alkane content, relative to fossil-basedhydrocarbon fuels. Thus, in some examples, sustainable aviation fuelsmay comprise either or both of a density of between 90% and 98% that ofkerosene and a calorific value of between 101% and 105% that ofkerosene.

Owing at least in part to the molecular structure of sustainableaviation fuels, sustainable aviation fuels may provide benefitsincluding, for example, one or more of a higher energy density; higherspecific energy; higher specific heat capacity; higher thermalstability; higher lubricity; lower viscosity; lower surface tension;lower freeze point; lower soot emissions; and, lower CO₂ emissions,relative to fossil-based hydrocarbon fuels (e.g. when combusted in thecombustion equipment 16). Accordingly, relative to fossil-basedhydrocarbon fuels, such as Kerosene, sustainable aviation fuels may leadto either or both of a relative decrease in specific fuel consumption,and a relative decrease in maintenance costs.

As depicted in FIGS. 4 and 7 , an aircraft 1 may comprise multiple fueltanks 50, 53; for example a larger, primary fuel tank 50 located in theaircraft fuselage, and a smaller fuel tank 53 a, 53 b located in eachwing. In other examples, an aircraft 1 may have only a single fuel tank50, and/or the wing fuel tanks 53 may be larger than the central fueltank 50, or no central fuel tank may be provided (with all fuel insteadbeing stored in the aircraft’s wings) – it will be appreciated that manydifferent tank layouts are envisaged and that the examples pictured areprovided for ease of description and not intended to be limiting.

FIG. 4 and FIG. 7 show an aircraft 1 with a propulsion system 2comprising two gas turbine engines 10. The gas turbine engines 10 aresupplied with fuel from a fuel supply system onboard the aircraft 1. Thefuel supply system of the examples pictured comprises a single fuelsource.

For the purposes of the present application the term “fuel source” meanseither 1) a single fuel tank or 2) a plurality of fuel tanks which arefluidly interconnected. Each fuel source is arranged to provide aseparate source of fuel i.e. a first fuel source may contain a firstfuel having a different characteristic or characteristics from a secondfuel contained in a second fuel source. First and second fuel sourcesare therefore not fluidly coupled to each other so as to separate thedifferent fuels (at least under normal running conditions).

In the present examples, the first (and, in these examples, only) fuelsource comprises a centre fuel tank 50, located primarily in thefuselage of the aircraft 1 and a plurality of wing fuel tanks 53 a, 53b, where at least one wing fuel tank is located in the port wing and atleast one wing fuel tank is located in the starboard wing for balancing.All of the tanks 50, 53 are fluidly interconnected in the example shown,so forming a single fuel source. Each of the centre fuel tank 50 and thewing fuel tanks 53 may comprise a plurality of fluidly interconnectedfuel tanks.

In another example, the wing fuel tanks 53 a, 53 b may not be fluidlyconnected to the central tank 50, so forming a separate, second fuelsource. For balancing purposes, one or more fuel tanks in the port wingmay be fluidly connected to one or more fuel tanks in the starboardwing. This may be done either via a centre fuel tank (if that tank doesnot form part of the other fuel source), or bypassing the centre fueltank(s), or both (for maximum flexibility and safety).

In another example, the first fuel source comprises wing fuel tanks 53and a centre fuel tank 50, while a second fuel source comprises afurther separate centre fuel tank. Fluid interconnection between wingfuel tanks and the centre fuel tank of the first fuel source may beprovided for balancing of the aircraft 1.

In some examples, the allocation of fuel tanks 50, 53 available on theaircraft 1 may be constrained such that the first fuel source and thesecond fuel source are each substantially symmetrical with respect tothe aircraft centre line. In cases where an asymmetric fuel tankallocation is permitted, a suitable means of fuel transfer is generallyprovided between fuel tanks of the first fuel source and/or between fueltanks of the second fuel source such that the position of the aircraft’scentre of mass can be maintained within acceptable lateral limitsthroughout the flight.

An aircraft 1 may be refuelled by connecting a fuel storage vessel 60,such as that provided by an airport fuel truck, or a permanent pipeline,to a fuel line connection port 62 of the aircraft, via a fuel line 61. Adesired amount of fuel may be transferred from the fuel storage vessel60 to the one or more tanks 50, 53 of the aircraft 1. Especially inexamples with more than one fuel source, in which different tanks 50, 53are to be filled with different fuels, multiple fuel line connectionports 62 may be provided instead of one, and/or valves may be used todirect fuel appropriately.

Aircraft generally refuel at multiple different airports, for example atthe beginning and end of a long-distance flight. Whilst there arestandards with which all aviation fuels must be compliant, as mentionedabove, different aviation fuels have different compositions, for exampledepending on their source (e.g. different petroleum sources, biofuels orother synthetic aviation fuels (often described as sustainable aviationfuels - SAFs), and/or mixtures of petroleum-based fuels, and otherfuels) and on any additives included (e.g. such as antioxidants andmetal deactivators, biocides, static reducers, icing inhibitors,corrosion inhibitors) and any impurities. As well as varying betweenairports and fuel suppliers, fuel composition of the available aviationfuel may vary between batches even for a given airport or fuel supplier.Further, fuel tanks 50, 53 of aircraft 1 are usually not emptied beforebeing topped up for a subsequent flight, resulting in mixtures ofdifferent fuels within the tanks - effectively a fuel with a differentcomposition resulting from the mixture.

The inventors appreciated that, as different fuels can have differentproperties, whilst still conforming to the standards, knowledge of thefuel(s) available to an aircraft 1 can allow more efficient, tailored,control of the aircraft 1, and more specifically of the aircraft’spropulsion system 2 (i.e. the one or more gas turbine engines 10 of theaircraft 1, and associated controls and components). Knowledge of thefuel can therefore be used as a tool to improve aircraft performance, somonitoring fuel composition can provide benefits.

In various examples, an active infinite summing approach may be taken,to keep track of varying fuel composition of a fuel within a fuel tank50, 53 with time, after multiple refills. For this approach, it isassumed that all aviation fuels are fully miscible, and that ahomogeneous mixture is formed within a fuel tank 50, 53 at least inaircraft operation (partitioning of fuels in-tank due to differences indensities may be seen in static tanks, when a less dense fuel is addedon top of a more dense fuel, but such partitioning would not be expectedto remain in flight, as movements of the tank and vibrations of thesystem will induce mixing). A record may be kept for each fuel tank, inexamples in which the aircraft 1 has multiple fuel tanks 50, 53.

Such an approach comprises obtaining one or more fuel characteristics ofany fuel already present in the fuel tank 50, 53 prior to refuelling.

As used herein, the term “fuel characteristics” refers to intrinsic orinherent fuel properties such as fuel composition, not variableproperties such as volume or temperature. Examples of fuelcharacteristics include one or more of:

-   i. the percentage of sustainable aviation fuel (SAF) in the fuel, or    an indication that the fuel is a fossil fuel, for example fossil    kerosene, or that the fuel is a pure SAF fuel;-   ii. parameters of a hydrocarbon distribution of the fuel, such as:    -   · the aromatic hydrocarbon content of the fuel, and optionally        also / alternatively the multi-aromatic hydrocarbon content of        the fuel;    -   the hydrogen to carbon ratio (H/C) of the fuel;    -   · % composition information for some or all hydrocarbons        present;-   iii. the presence or percentage of a particular element or species,    such as:    -   · the percentage of nitrogen-containing species in the fuel;    -   · the presence or percentage of a tracer species or trace        element in the fuel;    -   · naphthalene content of the fuel;    -   · sulphur content of the fuel;    -   · cycloparaffin content of the fuel;    -   · oxygen content of the fuel;-   iv. one or more properties of the fuel in use in a gas turbine    engine 10, such as:    -   · level of non-volatile particulate matter (nvPM) emissions or        CO₂ emissions on combustion;    -   · level of coking of the fuel;-   v. one or more properties of the fuel itself, independent of use in    an engine 10 or combustion, such as:    -   · thermal stability of the fuel (e.g. thermal breakdown        temperature); and    -   · one or more physical properties such as density, viscosity,        calorific value, freeze temperature, and/or heat capacity.

The fuel characteristics to be tracked may be selected based on whichproperties of the fuel are most relevant to changes which may be made tothe propulsion system 2.

The obtaining fuel characteristics of any fuel already present in thefuel tank 50, 53 prior to refuelling may comprise one or more of:

-   (i) physically and/or chemically detecting one or more features or    parameters of the composition of the fuel already present in the    fuel tank 50, 53 (this may allow direct detection of the fuel    characteristics, and/or may allow the fuel characteristics to be    determined using the detection results), and/or detecting one or    more tracer elements or compounds added to the fuel to facilitate    its identification (e.g. a dye);-   (ii) obtaining the result of an earlier determination performed    using an active infinite summing approach as described herein, for    example by retrieving one or more fuel characteristic values from a    local data store on-board the aircraft 1;-   (iii) receiving data, for example from an input provided at a user    interface, or data transmitted to the aircraft 1.

In some examples, multiple different methods may be performed to obtainthe fuel characteristics -for example, different methods may be used fordifferent characteristics, and/or different methods may be used for thesame characteristic as a check. In some examples, the obtaining the oneor more fuel characteristics of any fuel already present in the fueltank 50, 53 prior to refuelling may comprise obtaining stored fuelcharacteristic data, and chemically or physically detecting one or moreparameters of any fuel already present in the fuel tank 50, 53 prior torefuelling, and checking this against the stored fuel characteristicdata. The input to the calculating step described below may therefore beverified based on the one or more detected parameters. If there is amis-match between the stored fuel characteristic and the correspondingdetected parameter, an alert may be provided.

As mentioned above, for this approach it is generally assumed that thefuels are perfectly miscible, forming a homogeneous mixture within thetanks 50, 53. However, if there is any possibility of imperfect mixingof fuels within the tank 50, 53 (e.g. after a long period of no movementfor a fuel mix known to contain fuels of differing densities), fuelcomposition coming out of the fuel tank 50, 53 on its way to the engine10, 44 may be examined. If the measured, calculated, or otherwisedetermined fuel characteristics of fuel leaving the tank 50, 53 differfrom those of the homogeneous mixture expected to be in the tank 50, 53,a possible issue with imperfect mixing may be flagged in some scenarios(e.g. if there is a significant density difference between the fuelalready in the tank and the newly-added fuel, which could result instratification) instead of, or as well as, flagging possible errors inunderstanding of the overall tank contents.

Fuel characteristics may be detected in various ways, both direct (e.g.from sensor data corresponding to the fuel characteristic in question)and indirect (e.g. by inference or calculation from othercharacteristics or measurements, or by reference to data for a specificdetected tracer in the fuel). The characteristics may be determined asrelative values as compared to another fuel, or as absolute values. Forexample, one or more of the following detection methods may be used:

-   · The aromatic or cycloparaffin content of the fuel can be    determined based on measurements of the swell of a sensor component    made from a seal material such as a nitrile seal material.-   · Trace substances or species, either present naturally in the fuel    or added to act as a tracer, may be used to determine fuel    characteristics such as the percentage of sustainable aviation fuel    in the fuel or whether the fuel is kerosene.-   · Measurements of the vibrational mode of a piezoelectric crystal    exposed to the fuel can be used as the basis for the determination    of various fuel characteristics including the aromatic content of    the fuel, the oxygen content of the fuel, and the thermal stability    or the coking level of the fuel – for example by measuring the    build-up of surface deposits on the piezoelectric crystal which will    result in a change in vibrational mode.-   . Various fuel characteristics may be determined by collecting    performance parameters of the gas turbine engine 10 during a first    period of operation (such as during take-off), and optionally also    during a second period of operation (e.g. during cruise), and    comparing these collected parameters to expected values if using    fuel of known properties.-   . Various fuel characteristics including the aromatic hydrocarbon    content of the fuel can be determined based on sensor measurements    of the presence, absence, or degree of formation of a contrail by    the gas turbine 10 during its operation.-   · Fuel characteristics including the aromatic hydrocarbon content    can be determined based on a UV-Vis spectroscopy measurement    performed on the fuel.-   · Various fuel characteristics including the sulphur content,    naphthalene content, aromatic hydrogen content and hydrogen to    carbon ratio may be determined by measurement of substances present    in the exhaust gases emitted by the gas turbine engine 10 during its    use.-   · Calorific value of the fuel may be determined in operation of the    aircraft 1 based on measurements taken as the fuel is being burned –    for example using fuel flow rate and shaft speed or change in    temperature across the combustor 16.-   . Various fuel characteristics may be determined by making an    operational change arranged to affect operation of the gas turbine    engine 10, sensing a response to the operational change; and    determining the one or more fuel characteristics of the fuel based    on the response to the operational change.-   · Various fuel characteristics may be determined in relation to fuel    characteristics of a first fuel by changing a fuel supplied to the    gas turbine engine 10 from the first fuel to a second fuel, and    determining the one or more fuel characteristics of the second fuel    based on a change in a relationship between T30 and one of T40 and    T41 (the relationship being indicative of the temperature rise    across the combustor 16). The characteristics may be determined as    relative values as compared to the first fuel, or as absolute    values, e.g. by reference to known values for the first fuel.

(As used herein, T30, T40 and T41, and any other numbered pressures andtemperatures, are defined using the station numbering listed in standardSAE AS755, in particular:

-   . T30 = High Pressure Compressor (HPC) Outlet Total Temperature;-   . T40 = Combustion Exit Total Temperature;-   . T41 = High Pressure Turbine (HPT) Rotor Entry Total Temperature.)

In the examples currently being described, the amount of fuel present inthe tank 50, 53 prior to refuelling (e.g. mass, volume, and/or %full) isalso noted, for example being automatically detected and recorded incomputational storage/memory onboard the aircraft 1.

In addition to obtaining one or more fuel characteristics of any fuelalready present in the fuel tank 50, 53 prior to refuelling, one or morefuel characteristics of a fuel to be added to the fuel tank 50, 53 onrefuelling are also obtained.

The obtaining fuel characteristics of the fuel to be added to the fueltank 50, 53 on refuelling may comprise one or more of:

-   (i) physically and/or chemically detecting one or more features of    the composition of the fuel (e.g. in a testing unit off-wing, or as    the fuel is transported to a fuel tank on-wing, or indeed in use in    the gas turbine engine 10), so allowing direct detection of the fuel    characteristics or providing data from which they can be determined,    as mentioned above, and/or detecting one or more tracer elements or    compounds added to the fuel to facilitate its identification (e.g. a    dye);-   (iii) receiving data, for example from an input provided at a user    interface, or data transmitted to the aircraft, e.g. by scanning a    barcode associated with the fuel delivery.

As for obtaining fuel characteristics of fuel already in the tank 50,53, in some examples, multiple different methods maybe performed toobtain the fuel characteristics of the resultant fuel mixture - forexample, different methods may be used for different characteristics,and/or different methods may be used for the same characteristic as acheck. Any suitable detection method(s) as mentioned above may be used.Assuming that the assumption of even mixing applies, fuelcharacteristics of fuel leaving the tank 50, 53 and entering the engine10 may be determined as a means of validating the outcome of calculating2006 the fuel characteristics of the mixture.

In the examples currently being described, an amount of fuel added tothe tank 50, 53 on refuelling (e.g. explicitly by mass added, or volumeadded, and/or implicitly by change in mass, volume, or %full) is alsonoted (e.g. being automatically detected, or provided by a fuelsupplier, and recorded in computational storage/memory onboard theaircraft 1).

One or more fuel characteristics of the resultant fuel in the fuel tank50, 53 after refuelling are then calculated using the data on any fuelinitially in the tank, and the data on the fuel added to the tank. Suchcalculations may be performed for each fuel source separately - forexample, a first fuel source may be empty prior to refuelling and socontain only the new fuel, and a second fuel source may not be emptyprior to refuelling and may contain a mixture of the old and new fuelsafter refuelling. In such cases, burning fuel from the first fuel sourcein the gas turbine engine 10 may allow fuel characteristics of the newfuel to be determined, and fuel characteristics of the mixture in thesecond fuel source may then be calculated using the determinedcharacteristics for the new fuel, the blend percentage, and data on theolder fuel.

Current fuel characteristic data for the fuel in the tank may be stored,updating the recorded fuel characteristics of the fuel present in thefuel tank 50, 53 following each refuelling of the aircraft 1.Optionally, a continuous record of fuel compositions used with time maybe kept; alternatively, only a current fuel composition may be stored.

A fuel composition tracker 202 may be used to record and store fuelcomposition data, and optionally also to receive the fuelcharacteristics of the fuel to be added on refuelling, and calculateupdated fuel composition data. The fuel composition tracker 202 may beprovided as a separate fuel composition tracking unit 202, as shown inFIGS. 4 and 6 , or as a module built into the propulsion system 2,and/or as software and/or hardware incorporated into the pre-existingaircraft control systems, e.g. as a part of an electronic enginecontroller (EEC) 42.

In the example shown, two sensors 204 a, 204 b are provided, eacharranged to physically and/or chemically detect one or more features ofthe composition of the fuel being added to the fuel tank 50, 53 onrefuelling. The sensors 204 and the fuel composition tracker 202together may be described as a fuel composition tracking system 203, asshown in FIG. 6 .

In alternative examples, no such sensors 204 may be provided (forexample, a barcode associated with the fuel storage vessel 60 may beread and the corresponding data on the fuel provided to the fuelcomposition tracker 202), or more or fewer sensors may be provided.

Some examples may further comprise chemically or physically detectingone or more parameters of the resultant fuel in the fuel tank 50, 53after refuelling. The detected parameters may then be compared to one ormore of the calculated fuel characteristics, to verify the result. Ifthere is a mis-match between the calculated fuel characteristic and thecorresponding detected parameter, an alert may be provided.

An active infinite summing approach as described herein could be usedcontinually throughout the lifetime of an aircraft 1 (or betweenservices in which a fuel tank 50, 53 may be drained). However, it may bebeneficial to re-baseline the fuel composition data at intervals.

Re-baselining may comprise chemically and/or physically determining oneor more parameters of the fuel in the fuel tank 50, 53, and using thedetermined values to replace the stored fuel characteristics for thefuel in the fuel tank.

In some examples, the chemically and/or physically determining one ormore parameters of the fuel in the fuel tank 50, 53 for baselining maybe performed by extracting a sample of the fuel from the fuel tank foroff-wing testing; for example, it may be sent to a laboratory foranalysis, or provided to a ground-based testing station available at anairport. In other examples, on-wing, and optionally in situ, testingmethods may be used.

Re-baselining (i.e. chemically and/or physically determining one or moreparameters of the fuel in the fuel tank 50, 53 and using the determinedvalues to replace the stored fuel characteristics for the fuel in thefuel tank) may be performed in response to a trigger event. A triggerevent may be a threshold amount of time having elapsed since a previous(chemical and/or physical) determination of the one or more parametersof the fuel in the fuel tank, or a threshold number of refuelling eventsand/or flights having been reached since a previous (chemical and/orphysical) determination of the one or more parameters of the fuel in thefuel tank.

Additionally or alternatively, a trigger event may be the detection of adiscrepancy between one or more of the calculated characteristics and adetected parameter – for example when a calculated value differs from adetected value by an amount exceeding a threshold or tolerance value. Insome examples, an alert is provided (e.g. an audible and/or visiblealarm, and/or a message sent to the pilot or another party) on detectionof such a discrepancy – a decision may then be taken as to whether tore-baseline immediately, or to accept that uncertainty in fuelcomposition may mean being unable to operate most efficiently for thenext flight(s) until an opportunity to re-baseline becomes available.Smart control of the propulsion system 2 based on fuel characteristicsmay be disabled until the next re-baselining event.

Once the one or more fuel characteristics of the resultant fuel in thefuel tank 50, 53 after refuelling have been determined, the propulsionsystem 2 can be controlled based on the calculated fuel characteristics.

For example:

-   · An operating parameter of a heat management system of the aircraft    (e.g. a fuel-oil heat exchanger) may be changed, or the temperature    of fuel supplied to the combustor 16 of the engine 10 can be    changed.-   · When more than one fuel is stored aboard an aircraft 1, a    selection of which fuel to use for which operations (e.g. for    ground-based operations as opposed to flight, for low-temperature    start-up, or for operations with different thrust demands) may be    made based on fuel characteristics such as %SAF, nvPM generation    potential, viscosity, and calorific value. A fuel delivery system    may therefore be controlled appropriately based on the fuel    characteristics.-   · One or more flight control surfaces of the aircraft 1 may be    adjusted so as to change route and/or altitude based on knowledge of    the fuel.-   · The spill percentage of a fuel pump (i.e. the proportion of pumped    fuel recirculated instead of being passed to the combustor) may be    changed, e.g. based on the %SAF of the fuel. The pump and/or one or    more valves may therefore be controlled appropriately based on the    fuel characteristics.-   · Changes to the scheduling of variable-inlet guide vanes (VIGVs)    may be made based on fuel characteristics. The VIGVs may therefore    be moved, or a movement of the VIGVs be cancelled, as appropriate    based on the fuel characteristics.

A propulsion system 2 for an aircraft may therefore comprise a fuelcomposition tracking system 203 arranged to:

-   store 2002 current fuel characteristic data, the fuel characteristic    data comprising one or more fuel characteristics of fuel present in    the fuel tank 50, 53;-   obtain 2004 one or more fuel characteristics of a fuel added to the    fuel tank 50, 53 on refuelling; and-   calculate 2006 updated values for the one or more fuel    characteristics of the fuel in the fuel tank 50, 53 after    refuelling.

The obtaining 2004 of the one or more fuel characteristics of a fueladded to the fuel tank 50, 53 on refuelling may be performed before,during, or after the refuelling itself, for example using an off-wingtesting unit, fuel line sensors, or gas turbine engine performancesensors, respectively, or indeed receiving data electronically from athird party.

FIG. 6 shows an example of a fuel composition tracking system 203, inthe context of a refuelling event in which a fuel F is supplied to afuel tank 50, 53. Arrows with dashed lines in FIG. 6 indicate fuel flow,whereas solid lines indicate electronic communication.

The fuel composition tracking system 203 comprises a fuel compositiontracker 202. The fuel composition tracker 202 of the example beingdescribed comprises memory 202 a (which may also be referred to ascomputational storage) arranged to store the current fuel characteristicdata, and processing circuitry 202 c arranged to calculate updatedvalues for the one or more fuel characteristics of the fuel in the fueltank 50, 53 after refuelling. The calculated values may then replace thepreviously-stored fuel characteristic data in the memory, and/or may betime- and/or date-stamped and added to the memory. A log of fuelcharacteristic data with time may therefore be assembled.

The fuel composition tracker 202 of the example shown also includes areceiver 202 b arranged to receive data relating to fuel compositionand/or requests for fuel composition information. The fuel compositiontracker 202 of the example shown forms a part of, or is in communicationwith, an electronic engine controller (EEC) 42. The EEC 42 may bearranged to issue propulsion system control commands based on thecalculated fuel characteristics. It will be appreciated that an EEC 42may be provided for each gas turbine engine 10 of the aircraft 1, andthat the role played by the EEC in or for the fuel composition tracker202 may be just a small part of the functionality of the EEC. Indeed,the fuel composition tracker 202 may be provided by the EEC, or maycomprise an EEC module distinct from the engine’s EEC 42 in variousimplementations. In alternative examples, the fuel composition tracker202 may not comprise any engine control functionality, and may insteadsimply supply fuel composition data on demand, to be used as appropriateby another system. Optionally, the fuel composition tracker 202 maysupply a proposed change in engine control functionality for approval bya pilot; the pilot may then implement the proposed change directly, orapprove or reject the automatic making of the proposed change.

The method 2000 performed is illustrated in FIG. 5 . At step 2002,current fuel characteristic data, comprising one or more fuelcharacteristics of fuel present in the fuel tank 50, 53, are stored,optionally in memory of a fuel composition tracker 202. These data maybe provided for storage in any suitable way, for example being manuallyentered, e.g. via a graphical user interface in communication with thefuel composition tracker 202, electronically communicated to the fuelcomposition tracker 202, e.g. by wired or wireless communication from abarcode scanner following reading of a barcode (or equivalently anyother type of optically- or otherwise-readable code, such as a QR code),and/or determined from sensor data. It will be appreciated that, if thetank 50, 53 is currently empty, null values or equivalent may be stored.

At step 2004, characteristics of a fuel added to the fuel tank 50, 53 onrefuelling are determined. The determination may be performed by thefuel composition tracker 202 itself, e.g. by interpreting sensor data,or fuel characteristics determined elsewhere may be provided to the fuelcomposition tracker 202.

At step 2006, updated values for the one or more fuel characteristics ofthe fuel in the fuel tank 50, 53 after refuelling are calculated,optionally by the fuel composition tracker 202, using the stored fuelcharacteristic data (if any / if not null) and the obtained fuelcharacteristics.

The method 2000 may then be iterated on or after each refuelling event,returning to step 2002, with the updated values replacing the storedvalues (or being added to storage as part of a log), and proceedingaccordingly.

The method 2000 may further comprise controlling 2008 a propulsionsystem 2 of an aircraft 1 based on the calculated one or more fuelcharacteristics of the resultant fuel in the fuel tank 50, 53 afterrefuelling. Updated values may be used to influence that control aftereach refuelling event. The controlling 2008 may be performedautomatically in response to the determination of fuel properties, orafter approval by a pilot, following the pilot being notified of aproposed change. In some examples, the same method 2000 may includeautomatically making some changes, and requesting others, depending onthe nature of the change. In particular, changes which are “transparent”to the pilot - such as internal changes within engine flows which do notaffect engine power output and would not be noticed by a pilot - may bemade automatically, whereas any changes which the pilot would notice maybe notified to the pilot (i.e. a notification appearing that the changewill happen unless the pilot directs otherwise) or suggested to thepilot (i.e. the change will not happen without positive input from thepilot). In implementations in which a notification or suggestion isprovided to a pilot, this may be provided on a cockpit display of theaircraft and/or as an audible alarm, and/or sent to a separate devicesuch as a portable tablet or other computing device.

In examples in which an aircraft 1 has multiple fuel tanks 50, 53 whichare all fluidly linked such that the fuels in all of the tanks 50, 53are equivalent, a single set of fuel composition data may be stored andupdated. In examples in which an aircraft 1 has multiple fuel tanks 50,53 which are not fluidly linked, such that there may be differencesbetween fuels in the different tanks 50, 53, a separate set of fuelcomposition data may be stored and updated for each tank. In such cases,a determination of which fuel is being supplied to the gas turbineengine 10 may be performed prior to the making or suggestion of anypropulsion system control changes.

The inventors also appreciated that, as different fuels can havedifferent properties, whilst still conforming to the standards,knowledge of the fuel(s) available to an aircraft 1 can allow moreefficient, tailored, control of the flight profile. For example, a fuelwith a higher hydrogen to carbon ratio may allow the formation ofcontrails at higher threshold temperatures / lower altitudes, and achoice may be made to fly at a slightly higher altitude (e.g. 100 m to200 m higher), or to move to an adjacent discrete flight level(generally separated by 2000 feet vertical distance by current policies)to compensate for the otherwise-increased contrail formation.Additionally or alternatively, a different route may be selected totravel through slightly warmer or less humid air so as to reducecontrail formation. Knowledge of the fuel can therefore be used as atool to improve aircraft performance, for example by reducing contrailformation. It will be appreciated that contrail formation is describedhere by way of example only, and is not intended to be limiting. Advanceknowledge of the fuel for a flight of an aircraft 1 may therefore beused for advance planning and tailoring of details of the flightprofile, so improving environmental outcomes and/or aircraftperformance.

One or more fuel characteristics of a fuel arranged to be provided to agas turbine engine 10 of an aircraft may therefore be determined. Thefuel characteristics may include one or more of the fuel characteristicslisted above.

The determination may be performed in many different ways. For example:

-   · a bar code of a fuel to be added to a fuel tank 50, 53 of the    aircraft 1 may be scanned to read data of the fuel, or a tracer    substance (e.g. a dye) identified and fuel properties looked up    based on that tracer;-   · data may be manually entered, or transmitted to the aircraft 1;-   · a fuel sample may be extracted for ground-side analysis prior to    take-off;-   · fuel properties may be inferred from measurements of the    propulsion system activity during one or more periods of aircraft    operation, e.g. engine warm-up (including any engine use prior to    movement of the aircraft 1) taxi, take-off, climb and/or cruise;    and/or-   · one or more fuel properties may be detected in-flight, for example    using in-line sensors and/or other measurements.

The fuel characteristics may therefore be chemically and/or physicallydetected, determined from other sensed data, or otherwise determined.

In some examples, combinations of these techniques may be used todetermine and/or verify one or more fuel characteristics, for exampleusing one or more of the example detection techniques described above.

In some examples, such as those shown in FIGS. 4 and 7 described above,the aircraft 1 may have only a single fuel tank 50 (which may be in theform of a pair of linked wing tanks rather than a central tank), and/ormay have multiple fuel tanks 50, 53 which each contain the same fuel,and/or are fluidly linked, or fluidly connected to the gas turbineengine 10, such that only a single fuel type is supplied to the gasturbine engine 10 between refuelling events – i.e. the fuelcharacteristics may remain constant throughout a flight.

In other examples, the aircraft 1 may have a plurality of fuel tanks 50,53 which contain fuels of different compositions, and the propulsionsystem 2 may comprise an adjustable fuel delivery system, allowing aselection to be made of which tank(s), and therefore what fuel/fuelblend, to use. In such examples, the fuel characteristics may vary overthe course of a flight, and a specific fuel or fuel blend may beselected to improve operation at certain flight stages or in certainexternal conditions.

Once one or more fuel characteristics have been determined, a flightprofile may be selected, changed, or adjusted based on those fuelcharacteristics. In many examples, external data - for example weatherdata such as humidity and temperature data, and time data such as dayvs. night -may be used in combination with the determined fuelcharacteristics to select or adjust the flight profile.

For example, the method implemented may comprise receiving forecastweather conditions for an intended route of the aircraft 1. Thesereceived forecast weather conditions may be used to make or influencechanges in planned route and/or altitude, or used to guide planning ofthe route and/or altitude.

As used herein, the term “flight profile” refers to the operationalcharacteristics (e.g. height/altitude, power setting, flight path angle,airspeed, and the like) of an aircraft 1 as it flies along a flighttrack, and also to the trajectory/flight track (route) itself. Changesof route are therefore included in the term “flight profile” as usedherein.

In examples in which some or all of the fuel characteristics areinferred from measurements of the propulsion system activity duringearly stages of aircraft operation, for example start-up / enginewarm-up, taxi, take-off and climb, or otherwise measured on-wing, theflight profile during cruise may be adjusted even if knowledge of thefuel characteristics is not available in time to guide the flightprofile in the earlier stages of operation.

In examples in which some or all of the fuel characteristics aredetermined prior to take-off (e.g. on refuelling, or by analysis of thepropulsion system 2 during taxiing), the flight profile for take-offand/or climb may also be adjusted; for example a take-off time,direction, and/or steepness of ascent may be selected to avoidhigher-humidity regions or time periods.

In either case, the future course and/or operational characteristics ofthe aircraft 1 can be adjusted based on the determined fuelcharacteristics - advance planning of how to control the aircraft 1based on the available fuel, and in particular of a specific flighttrajectory (in particular, route and altitude), can therefore beperformed.

A propulsion system 2 for an aircraft 1 may therefore comprise a fuelcomposition determination module 210 arranged to determine 2052 one ormore fuel characteristics of the fuel arranged to be provided to the gasturbine engine 10; and a flight profile adjustor 212 arranged to proposeor initiate a change to the planned flight profile based on the one ormore fuel characteristics of the fuel.

In some examples, the fuel characteristics may be determined a period oftime before a flight is due to commence and planned changes to theflight profile may then be proposed to a pilot and/or air trafficcontroller or other authority (optionally via an automated system), soas to obtain approval of the changes prior to take-off. In otherexamples, the changes made may be minor enough to not require sign-offfrom air traffic control, nor indeed from the pilot, and may beimplemented automatically. An automated notification or proposal of thechange may be provided to air traffic control and/or to the pilot, asappropriate. The notification or proposal may be provided on a cockpitdisplay of the aircraft 1, and/or sent to a separate device such as aportable tablet or other computing device.

If the fuel composition is known far enough in advance of the flight,one or more appropriate authorities with whom a flight plan has beenfiled may also be notified of a change in flight plan, or a new flightplan may be filed with them.

Examples in which fuel composition may be known for certain well inadvance of a flight might include cases where an aircraft 1 carriesenough fuel for its current flight from a first airport to a second, andfor its next intended flight onwards (or back to the first airport) fromthe second airport. A decision to carry excess fuel, rather thanrefuelling at the second airport, may be taken to enable a fastturnaround at the second airport or to avoid high fuel prices at thesecond airport, or indeed if a fuel composition available at the secondairport is not desirable. Thus, after loading fuel at the first airport,any changes that might need to be made for the second flight from thesecond airport may be determined before or during the first flight, anda filed flight plan for the second flight may be replaced or adjustedaccordingly.

Various different locations and types of fuel composition determinationmodule 210 may be used depending on when, where, and how, fuelcomposition is to be determined. For example, fuel composition may bedetermined for fuel when it first enters the aircraft 1, via a fuel lineconnection port 62, with a fuel composition determination module 210 aoptionally located along a fuel supply line within the aircraft 1,leading from the fuel line connection port 62 to a tank 50, 53. It willbe appreciated that if fuel composition data are provided to theaircraft 1, manually or electronically, on refuelling, no sensing ormeasurement of the incoming fuel may be required and the fuelcomposition determination module 210 a may be located wherever isconvenient and arranged to receive that data. Alternatively (oradditionally), fuel composition may be determined for fuel within a tank50, 53, with a fuel composition determination module 210 b optionallylocated in or adjacent a tank 50, 53 within the aircraft 1.Alternatively (or additionally), fuel composition may be determined forfuel approaching a combustor 16 of the gas turbine engine 10, with afuel composition determination module 210 c optionally located near afuel feed line from the fuel tank 50, 53 to the combustor 16, or usingany other approach described above. In some examples, one or more fuelcharacteristic sensors may be provided integrally with processingcircuitry and/or memory of the fuel composition determination module210. In alternative examples, one or more fuel characteristic sensorsmay be located remotely from, and be in communication with, processingcircuitry and/or memory of the fuel composition determination module210. In alternative or additional examples, one or more fuelcharacteristics may be communicated to the fuel compositiondetermination module 210, which may in such cases not comprise one ormore sensors. The location of processing circuitry and/or memory of thefuel composition determination module 210 may vary accordingly. It willbe appreciated that, whilst FIG. 7 shows three fuel compositiondetermination modules 210 a, b, c, these are provided to demonstratepossible locations only - just a single fuel composition determinationmodule 210 may be provided in other examples. A second fuel compositiondetermination module 210 may be provided in some examples, to provide aredundant check, and/or to determine fuel composition of a differentfuel source, in examples in which an aircraft 1 comprises one or morefuel tanks 50, 53 which are not fluidly linked.

In some examples, fuel composition may be detected when the aircraft 1is refuelled. The fuel composition determination module 210 a in suchexamples may be or comprise a fuel composition tracker 202.

In such examples, a fuel composition tracker 202 as shown in FIG. 6 maybe used to record and store fuel composition data, and optionally alsoto receive the fuel characteristics of the fuel to be added onrefuelling, and calculate updated fuel composition data as describedabove. Arrows with dashed lines in FIG. 6 indicate fuel flow, whereassolid lines indicate electronic communication.

The active infinite summing approach described above may therefore beused in conjunction with the flight profile adaptation approachcurrently being described, or either approach may be used in isolation.The flight profile adjustor 212 shown in FIG. 6 may not be present inexamples in which the flight profile adaptation approach currently beingdescribed is not implemented.

As mentioned above with respect to the earlier examples, the fuelcomposition tracker 202 may be provided as a separate fuel compositiontracking unit 202, as shown in FIG. 6 , or as a module built into thepropulsion system 2, and/or as software and/or hardware incorporatedinto the pre-existing aircraft control systems, e.g. the EEC 42.

In the example shown, two sensors 204 a, 204 b are provided, eacharranged to physically and/or chemically detect one or more features ofthe fuel being added to the fuel tank 50, 53 on refuelling. The sensors204 and the fuel composition tracker 202 together may be described as afuel composition tracking system 203, as shown in FIG. 6 .

The fuel composition tracking system 203 comprises a fuel compositiontracker 202. The fuel composition tracker 202 of the example beingdescribed comprises memory 202 a arranged to store the current fuelcharacteristic data, and processing circuitry 202 c arranged tocalculate updated values for the one or more fuel characteristics of thefuel in the fuel tank 50, 53 after refuelling. The calculated values maythen replace the previously stored fuel characteristic data in thememory, and/or may be time- and/or date-stamped and added to the memory.A log of fuel characteristic data with time may therefore be assembled.

The fuel composition tracker 202 of the example shown also includes areceiver 202 b arranged to receive data relating to fuel compositionand/or requests for fuel composition information. The fuel compositiondetermination module 210 may therefore comprise a receiver 202 barranged to receive data relating to fuel composition, from which one ormore fuel characteristics can be determined (either directly byextraction, or by calculation, optionally in conjunction with data fromanother source).

The fuel composition determination module 210 may therefore comprise, orhave access to the output of, one or more sensors 204 arranged toprovide data relating to one or more fuel characteristics. The sensordata may provide one or more fuel characteristics directly, or may allowone or more fuel characteristics to be obtained by calculation,optionally in conjunction with data from another source. In alternativeexamples, no such sensors 204 may be provided (for example, a barcodeassociated with the fuel storage vessel 60 may be read and thecorresponding data on the fuel provided to the fuel composition tracker202), or more or fewer sensors may be provided.

Data from the fuel composition tracker 202 may be used to change theplanned flight profile, based on the one or more fuel characteristics.

A flight profile adjustor 212 may be used to change the planned flightprofile based on the one or more fuel characteristics of the fuel, basedon data provided by the fuel composition tracker 202 and optionally alsoother data. The flight profile adjustor 212 may be provided as aseparate flight profile adjusting unit 212 built into the propulsionsystem 2, and/or as software and/or hardware incorporated into thepre-existing aircraft control systems, such as the EEC 42. Fuelcomposition tracking abilities (e.g. tracker 202) may be provided aspart of the same unit or package.

The flight profile control method 2050 performed is illustrated in FIG.8 . At step 2052, one or more fuel characteristics of the fuel arrangedto be provided to the gas turbine engine 10 are determined, optionallyusing any of the methods described above.

At step 2054, the flight profile of the aircraft 1 is changed based onthe one or more fuel characteristics. The change in flight profile maybe or comprise a change to one or more of trajectory, route, angle ofattack, and altitude.

A flight profile adjustor 212 may be used to initiate and/or effect thechange in flight profile. In some examples, the flight profile adjustor212 may change the flight profile itself, and, in some implementations,may additionally control implementation of that change, for examplerecording a planned change and then providing a command to one or moreflight control surfaces of the aircraft 1 so as to change altitude atthe appropriate time. In other examples, the flight profile adjustor 212may seek approval for the planned change, and/or may not itself sendinstructions to cause the change in flight profile. The flight profileadjustor 212 may therefore provide a notification or suggestion of aproposed change in flight profile to the pilot and/or another authorityregarding the planned change, for approval. A notification or suggestionmay be provided to a pilot on a cockpit display of the aircraft, and/orsent to a separate device such as a portable tablet or other computingdevice, for example. In some examples, the same flight profile adjustor212 may automatically make some changes, and request approval of others,depending on the nature of the change (e.g. whether or not the plannedchange is significant enough to need authorisation from air trafficcontrol, or another authority). The flight profile adjustor 212 maytherefore propose and/or initiate a change 2054 to a flight profile ofthe aircraft 1 based on the at least one fuel characteristic.

The inventors also appreciated that knowledge of the one or more fuelcharacteristics selected and determined in any of the ways describedabove may be used to suggest, guide, or make in-flight adjustments tothe propulsion system 2, so as to further improve aircraft performance.For example, a fuel with a higher heat capacity may be used for moreengine cooling than a fuel with a lower heat capacity, and a fuel with ahigher calorific value may allow a lower flow rate of fuel to besupplied to the combustor for the same power output. Knowledge of thefuel can therefore be used as a tool to improve aircraft performance inflight. As compared to the advance planning and flight profile changesdescribed above, real-time or near-real-time decisions may be made andimplemented, and these decisions may only affect the internal workingsof the engine 10 rather than changing route and/or altitude, forexample.

In some examples, the aircraft 1 may have only a single fuel tank 50,and/or may have multiple fuel tanks 50, 53 which each contain the samefuel, and/or are fluidly linked, or fluidly connected to the gas turbineengine 10, such that only a single fuel type is supplied to the gasturbine engine 10 between refuelling events – i.e. the fuelcharacteristics may remain constant throughout a flight. In suchexamples, fuel properties do not change during a flight, but externalconditions (e.g weather, altitude) and internal conditions (e.g. thrustdemand) do, and changes may be made (i) initially when the fuelcharacteristics are first determined or processed, and/or (ii) based onwhat is appropriate for that fuel given condition changes.

In other examples, the aircraft 1 may have a plurality of fuel tanks 50,53 which contain fuels of different compositions, and the propulsionsystem 2 may comprise an adjustable fuel delivery system, allowing aselection to be made of which tank(s) 50, 53, and therefore whatfuel/fuel blend, to use. In such examples, the fuel characteristics mayvary over the course of a flight, and a specific fuel or fuel blend maybe selected to improve operation at certain flight stages or in certainexternal conditions. In such examples, changes to propulsion systemcontrol may also be made when the fuel changes e.g. due to adetermination that one fuel is nearly running out, or to the selectionof a different fuel or fuel blend. (It will be appreciated that, ingeneral, a fuel system may be arranged to never let a tank 50, 53 runcompletely dry, as that could lead to an engine 10 flaming-out –however, a tank may be allowed to be fully emptied if its fuel is beingprovided as part of a blend; one or more other fuels of the blend mayhave their flow rate increased to ensure the engine 10 is never short offuel.) A change of fuel may therefore be a response to a propulsionsystem control change, and may provoke one or more further propulsionsystem control changes.

In examples in which direct detection is used for one or more fuelcharacteristics, or in which the fuel characteristics are calculatedfrom detected parameters, the detection may be performed in the or eachtank 50, 53 (and fuel characteristics for a resultant fuel blend fromdifferent tanks may then be calculated where appropriate), and/or onapproach to engine 10, e.g. in a pipe containing a blended mix frommultiple tanks. In some examples, the detection may be performed on thefuel immediately before entering engine 10, or more specifically thecombustor 16, to ensure that the correct fuel/fuel blend is identifiedand that the data are as up-to-date as possible (near real-time).

Once one or more fuel characteristics have been determined for fuelcurrently being provided to the gas turbine engine 10, control of thepropulsion system 2 may be adjusted based on the determined fuelcharacteristics.

Additional data may be used in conjunction with the determined fuelcharacteristics to adjust control of the propulsion system 2. Forexample, data of current conditions around the aircraft 1 may bereceived (either from a provider, such as a third-partyweather-monitoring company, or from on-board detectors). These receiveddata (e.g. weather data, temperature, humidity, presence of a contrail,etc.) may be used to make or influence changes in propulsion systemcontrol. Instead of, or as well as, using “live” or near-live weatherdata, forecast weather data for the aircraft’s route may also be used toestimate current conditions.

Examples of propulsion system changes which may be made based on thefuel characteristics include any or all of the control examplesdescribed above, such as adjusting VIGV scheduling.

A propulsion system 2 for an aircraft 1 may therefore comprise a fuelcomposition determination module 210 arranged to determine 2052 one ormore fuel characteristics of the fuel arranged to be provided to the gasturbine engine 10; and an electronic engine controller 42 arranged toissue propulsion system control commands based on the determined fuelcharacteristics. The fuel composition tracker 202 of the example shownmay be part of, or have access to, an electronic engine controller (EEC)42 arranged to issue propulsion system control commands based on thefuel characteristics. In some cases, the EEC may issue recommendationsfor pilot approval (or approval by another authority), and may thenissue a propulsion system control command subject to that approval. Itwill be appreciated that an EEC 42 may be provided for each gas turbineengine 10 of the aircraft 1, and that the role played by the EEC for thefuel composition tracker 202 may be just a small part of thefunctionality of the EEC. Indeed, the fuel composition tracker 202 maybe provided by the EEC, or may comprise an EEC module distinct from theengine’s EEC 42 in various implementations. In alternative examples, thefuel composition tracker 202 may not comprise any engine controlfunctionality, and may instead simply supply fuel composition data ondemand, to be used as appropriate by another system.

Various different locations and types of fuel composition determinationmodule 210 and associated fuel characteristic sensors may be useddepending on when, where, and how, fuel composition is to be determined.For example, as described above with respect to the flight profilecontrol method 2050 shown in FIG. 8 .

In some examples, fuel composition may be detected when the aircraft 1is refuelled. The fuel composition determination module 210 a in suchexamples may be or comprise a fuel composition tracker 202.

The active infinite summing approach described above may therefore beused in conjunction with the flight profile adaptation approachdescribed above, and/or with the in-flight adjustment approach currentlybeing described, or any of the three may be used in isolation. Asmentioned above, the flight profile adjustor 212 shown in FIG. 6 may notbe present in examples in which the flight profile adaptation approachdescribed above is not implemented.

The fuel composition tracker 202 and fuel composition tracking system203 may be as described above.

Data from the fuel composition tracker 202 may be used to change theplanned flight profile and/or to guide or make in-flight adjustments tothe propulsion system 2, based on the one or more fuel characteristics.

A propulsion system 2 for an aircraft 1 may therefore comprise a fuelcomposition tracker 202, or other fuel composition determination module210, arranged to record and store fuel composition data, and optionallyalso to receive the fuel characteristics of the fuel to be added onrefuelling, and calculate updated fuel composition data. The fuelcomposition determination module 210 may be provided as a separate fuelcomposition tracking unit built into the propulsion system, and/or assoftware and/or hardware incorporated into the pre-existing aircraftcontrol systems.

Data from the fuel composition determination module 210 may be used toadjust control of the propulsion system 2, based on the one or more fuelcharacteristics.

A propulsion system controller 42, also referred to as an electronicengine controller 42, may be used to adjust control of the propulsionsystem 2 based on the one or more fuel characteristics of the fuel,using data provided by the fuel composition determination module 210 andoptionally other data. It will be appreciated that the propulsion systemcontroller 42 may control propulsion system elements which may or maynot be considered as components of the engine 10 itself, such as one ormore flight control surfaces. The term “electronic engine controller”(EEC) 42 as used synonymously herein is not intended to be limited inthat sense. The propulsion system controller 42 may be provided as aseparate propulsion system controlling unit 42 built into the propulsionsystem 2, as a part of the fuel composition determination module 210,and/or as software and/or hardware incorporated into the pre-existingaircraft control systems. Fuel composition tracking abilities may beprovided as part of the same unit or package.

The propulsion system controller 42 may make changes to the propulsionsystem 2 directly or may provide a notification or suggestion to thepilot (or other authority) regarding the change, for approval. Anotification or suggestion may be provided to a pilot on a cockpitdisplay of the aircraft, and/or sent to a separate device such as aportable tablet or other computing device. In some examples, the samepropulsion system controller 42 may automatically make some changes, andrequest others, depending on the nature of the change. In particular, asmentioned above, changes which are “transparent” to the pilot – such asinternal changes within engine flows which do not affect engine poweroutput and would not be noticed by a pilot - may be made automatically,whereas any changes which the pilot would notice may be notified to thepilot (i.e. a notification appearing that the change will happen unlessthe pilot directs otherwise) or suggested to the pilot (i.e. the changewill not happen without positive input from the pilot).

The method 2051 performed is illustrated in FIG. 9 . At step 2052, oneor more fuel characteristics of the fuel arranged to be provided to thegas turbine engine 10 are determined. The fuel characteristics may be orcomprise any of those listed above, and may be selected based on whichcharacteristics have the most significant effects on optimal propulsionsystem control. At step 2056, the propulsion system 2 of the aircraft 1is controlled based on the one or more fuel characteristics. The controlactions taken may be or comprise any of those listed above.

A propulsion system controller 42 may therefore be used to initiateand/or effect the control of the propulsion system 2. In some examples,the propulsion system controller 42 may make a change automatically, forexample by providing a command to cause a change in position of one ormore variable inlet guide vanes of the aircraft propulsion system 2 inresponse to an assessment of fuel characteristics (and optionally ofother conditions). In other examples, the propulsion system controller42 may not automatically send instructions to control the propulsionsystem 2, but may instead provide a proposed change in propulsion systemcontrol for approval, based on one or more fuel characteristics.

The inventors also appreciated that, as different fuels can havedifferent properties whilst still conforming to the standards, knowledgeof the fuel(s) available to an aircraft 1 can allow more efficient,tailored, control of the propulsion system 2. For example, changing to afuel with a higher calorific value may allow for a constant rate of fuelsupply to the combustor 16 whilst still providing a higher power output.Selection of a specific fuel based on the intended or current aircraftoperations can therefore be used as a tool to improve aircraftperformance. In particular, calorific value of a fuel may be considered.

Below, this approach is described with respect to two aircraft fuelsource arrangements different from that shown in FIG. 4 . It will beappreciated that any of the approaches described herein may be used withany suitable fuel supply system, and that the examples pictured anddescribed in detail are not intended to be limiting.

In particular, as depicted in FIG. 10 and FIG. 14 , an aircraft 1 maycomprise multiple fuel tanks 50, 52, 53; for example, a first fuel tank52 and a second, larger, fuel tank 50, each located in the aircraftfuselage, and a smaller fuel tank 53 a, 53 b located in each wing. Inother examples, only two fuel tanks 50, 52, or more fuel tanks, may beprovided. Fuel tank sizes, shapes, and locations may vary; for example,all fuel may be stored in tanks 53 in the wings.

FIG. 10 shows an aircraft 1 with a propulsion system 2 comprising twogas turbine engines 10. The gas turbine engines 10 are supplied withfuel from a fuel supply system onboard the aircraft 1. The fuel supplysystem of the example pictured comprises two fuel sources. Each of thefuel sources is arranged to provide a separate source of fuel i.e. theyare fluidly isolated and the first fuel source may contain a first fuelhaving a different characteristic or characteristics from a second fuelcontained in the second fuel source. For example, the fuels may havedifferent compositions and/or different origins, e.g. one being afossil-derived fuel such as Jet-A, another being paraffinic SAF,non-paraffinic SAF, a paraffinic SAF with a different composition, or ablend. First and second fuel sources are therefore not fluidly coupledto each other, so as to separate the different fuels (at least undernormal running conditions). A fuel source may be a single tank or madeup of multiple fluidly interconnected tanks, and may be referred to as afuel tank even when it in fact comprises multiple interlinked tanks.

In the present example, the first fuel source is the first fuel tank 52.In other examples, the first fuel source may comprise multipleinterlinked tanks.

In the present example, the second fuel source comprises a centre fueltank 50, located primarily in the fuselage of the aircraft and aplurality of wing fuel tanks 53 a, 53 b, where at least one wing fueltank is located in the port wing and at least one wing fuel tank islocated in the starboard wing for balancing. All of the tanks 50, 53except the first fuel tank 52 are fluidly interconnected in the exampleshown in FIG. 10 , so forming a single, second, fuel source. Each of thecentre fuel tank and the wing fuel tanks may comprise a plurality offluidly interconnected fuel tanks.

In another example, the wing fuel tanks 53 a, 53 b may not be fluidlyconnected to the central tank 50, so forming a separate, third, fuelsource. For balancing purposes, one or more fuel tanks in the port wingmay be fluidly connected to one or more fuel tanks in the starboard wingas described above. In the example of FIG. 10 , however, fluidinterconnection between wing fuel tanks 53 and the centre fuel tank 50of the second fuel source is provided for balancing of the aircraft 1.

The example shown in FIG. 14 is generally similar to that shown in FIG.10 , but the differences are described below.

In the example shown in FIG. 14 , the first fuel tank 52 is smaller thanthe second fuel tank 50. The first fuel tank 52 of this implementationis located further towards the rear of the fuselage. The first fuel tank52 may therefore be used as a trim tank 52 in flight – it will beappreciated that, as is known in the art, a trim tank 52 can be used toprovide control of the centre of gravity in the longitudinal axis of theaircraft 1; the aircraft is “trimmed” by pumping fuel into (or out of) atrim tank. One or more pumps, valves, sensors, controls, and similar maybe provided to control the trimming operation. In other implementations,the first fuel tank 52 may not be a trim tank; its connections to otherfuel tanks 50, 53, may differ or not be present in such otherimplementations. A trim tank 52 is therefore fluidly connected to atleast one other fuel tank 50, 53 of an aircraft 1 so as to allowtrimming to be performed in flight. This is a controllable fluidconnection, so the first tank 52 can be isolated from the other tanks50, 53 and used as a separate fuel source when desired.

In addition to the propulsion system 2 described with respect to FIG. 10, a power system 4 of the implementation shown in FIG. 14 includes anAuxiliary Power Unit (APU) 44. The APU 44 is a gas turbine enginesmaller than those 10 on the wings of the aircraft 1, and is arranged toprovide electrical power to systems of the aircraft 1; for example,lighting, heating, air conditioning and/or similar. The APU 44 may be,for example, an APU in Honeywell’s 331 Series, such as the HGT1700auxiliary power unit (APU). In some implementations, the APU 44 may becertified for in-flight use; in other implementations, it may becertified for ground use only. An aircraft APU 44 is generally arrangedto be started using one or more aircraft batteries so as to provideelectrical power as well as optionally bleed air for air conditioningand for engine start. The APU 44 of the implementation shown is locatedtowards the rear of the fuselage, and is not arranged to provide anypropulsive power to the aircraft 1. In alternative implementations, theAPU may be differently located (e.g. within a nacelle 21 of the aircraft1), and/or may provide some propulsive power.

In the example shown in FIG. 14 , the first fuel tank 52 is arranged tosupply fuel to the APU 44. In this example, the first fuel tank 52 isalso arranged to supply fuel directly to the main gas turbine engines10, although these connections may not be present in someimplementations, or, alternatively, the first fuel tank 52 may bearranged to supply fuel directly to the main gas turbine engines 10 andnot to the APU 44 in some implementations.

In alternative implementations, the first fuel tank 52 may be adedicated APU fuel tank, and may not be fluidly interconnected to themain gas turbine engines 10, nor to any other fuel tank. In some suchexamples, a fuel manager 214 as described below may not be used for theAPU fuel supply.

In various implementations, a plurality of fuel line connection ports 62may be provided, optionally to facilitate supply of different fuels todifferent tanks/fuel sources 50, 52, 53. Alternatively or additionally,a fuel supply management system may direct incoming fuel from the sameport 62 to different tanks, as appropriate. In particular, in theexamples being described, the first tank 52 can be fuelled directly froma fuel supply rather than having to be filled by transfer from anotherfuel tank 50, 53 of the aircraft 1. In FIG. 14 , internal fuel linesfrom the port(s) 62 to the tanks are not shown, for clarity. In FIG. 10, one internal fuel line is shown (to the larger tank 50); it will beappreciated that at least a second fuel line to the first fuel tank 52would generally also be provided but that is not shown for clarity.

In examples of the present invention, the aircraft 1 has a plurality offuel tanks 50, 52, 53, and in particular, at least two different fuelsources and optionally more. At least two of the fuel tanks are arrangedto contain different fuels – i.e. fuels with at least one difference inthe fuel characteristics –and in particular, to contain fuels withdifferent calorific values.

One or more fuel characteristics of each fuel stored onboard theaircraft 1 may be determined. The determined fuel characteristics mayinclude one or more of the example fuel characteristics listed above.

The determination may be performed in many different ways. For example:

-   a barcode of a fuel to be added to a fuel tank 50, 52, 53 of the    aircraft 1 may be scanned to read data of the fuel, or a tracer    substance (e.g. a dye) identified and fuel properties looked up    based on that tracer;-   · data may be manually entered, or transmitted to the aircraft 1;-   a fuel sample may be extracted for ground-side analysis prior to    take-off;-   · fuel properties may be inferred from measurements of the    propulsion system activity during one or more periods of aircraft    operation, e.g. engine warm-up, taxi, take-off, climb and/or cruise;    and/or-   one or more fuel properties may be detected in-flight, for example    using in-line sensors and/or other measurements, and any of the    detection approaches described above.

The fuel characteristics may therefore be chemically and/or physicallydetected, or otherwise determined, by any suitable means describedherein. In some examples, combinations of these techniques may be usedto determine and/or verify one or more fuel characteristics.

Calorific values (also referred to as heating values) of the fuels maybe directly determined – for example by measuring or inferring theenergy released when a certain volume or mass of the fuel is combustedin the gas turbine engine 10 – or calculated from other fuel parameters;e.g. looking at the hydrocarbon distribution of the fuel and thecalorific value of each constituent hydrocarbon type. Alternatively, oradditionally so as to provide verification, the calorific value may bedetermined using external data, such as a look-up table for a tracersubstance in the fuel, or data encoded in a barcode associated with thefuel.

In some examples, every fuel tank 50, 52, 53 of the aircraft 1 may bearranged to contain a fuel with a different calorific value; i.e. everyfuel tank may be a separate fuel source.

As used herein, the term “calorific value” denotes the lower heatingvalue (also known as net calorific value) of the fuel, unless otherwisespecified. The net calorific value is defined as the amount of heatreleased by combusting a specified quantity of the fuel, generally inunits of J/kg, assuming that the latent heat of vaporisation of water inthe reaction products is not recovered (i.e. that produced water remainsas water vapour after combustion).

In some examples, two or more tanks 50, 52 of the aircraft may bearranged to contain fuels with a different type or proportion of asustainable aviation fuel, the fuels having different calorific values.

The propulsion system 2 comprises an adjustable fuel delivery system220, allowing a selection to be made of which source(s)/tank(s) 50, 52,53, and therefore what fuel or fuel blend, to use. In such examples, thefuel characteristics may vary over the course of a flight, and aspecific fuel or fuel blend may be selected to improve operation atcertain flight stages or in certain external conditions.

A first fuel tank 52 of the plurality of fuel tanks 50, 52, 53 may havea higher proportion of sustainable aviation fuel (SAF) than a secondfuel tank 50 of the plurality of fuel tanks and may have a highercalorific value than a fossil-based fuel, or SAF-fossil blend, inanother, second, fuel tank. More fuel from the second tank 50 may beused at cruise and more fuel from the first tank 52 may be used atoperating points with higher power demands (e.g. take-off and climb).

In other examples, the first fuel tank 52 may contain a fuel with alower calorific value than that in another tank 50. More fuel from thefirst tank 52 may be used at cruise and more fuel from the second tank50 may be used at operating points with higher power demands (e.g.take-off and climb).

One fuel tank 52 of the plurality of fuel tanks may be arranged tocontain only a fuel which is a sustainable aviation fuel (SAF) – thattank may contain 100% pure SAF, or SAF with one or more additives, butdoes not contain any fossil-based fuel. (As used herein, “SAF” means apure sustainable aviation fuel, containing no fuel of a fossil/petroleumorigin (but optionally one or more additives, e.g. an icing inhibitor);the term “SAF blend” or “blended fuel” may be used for a mixtureincluding both SAF and fuel of a petroleum origin.) The SAF in that tank52 may be selected such that the propulsion system 2 can be run on thatfuel alone (for example, for ground operations, or in case of anemergency in flight, or if/when fuel regulations change such that flighton SAF fuel alone is generally permitted).

In such examples, the fuel tank 52 containing the sustainable aviationfuel only may therefore be arranged to be used to power the aircraft 1when the aircraft is performing operations on the ground. Optionally,all fuel, or at least the majority of fuel, used for ground operationsmay be arranged to be taken from the fuel tank 52 containing thesustainable aviation fuel, for example to meet airport requirements foremissions and/or use of SAF. A SAF blend fuel with a high %SAF may beused in place of SAF in other implementations.

It will be appreciated that use of SAF (either alone or in a mixed fuel)may provide a significant reduction in non-volatile particulate matter(nvPM) emissions at idle conditions – the percentage reduction may begreater than 90% at those conditions in some cases. It is thought thatthe percentage reduction in nvPM for SAF use is greater at idle than athigh power demands, as soot (nvPM) creation is linked more closely tothe fuel aromatic content at those low power conditions compared to athigher power demands where other soot formation mechanisms come intoplay -SAFs generally have a lower aromatic content thanpetroleum-derived aviation fuels. As such, if a total amount of the SAFis limited, using the SAF for ground-based operations / operationsaround the airport instead of elsewhere in a flight cycle may provide anincreased benefit in reduction of nvPM production. Additionally, airportair quality may be improved. Similarly, in-flight, a larger benefit fromusing SAF may be found if the SAF is used for lower-power parts of theflight envelope.

For the same reasons, SAF may be selected to be used in an auxiliarypower unit (APU) of the aircraft 1 at the gate of an airport.

However, SAFs often have a higher calorific value than traditional jetfuels - in such cases, different control of fuel input to the gasturbine engine 10 may therefore be used for ground-based operations(where reductions in nvPM may be prioritised) as compared to in flight(where matching calorific value to thrust may be prioritised), if only alimited amount of SAF is available. In scenarios in which fuels withsimilar nvPM properties but different calorific values are availableonboard an aircraft 1, the same control may be used throughoutoperation; both for ground-based operations and in flight.

In examples with a SAF-only tank 52, that fuel tank 52 may be smallerthan the one or more other fuel tanks 50, 53, for example, the firstfuel tank 52 may represent 3% to 20%, and optionally 5% to 10%, of thetotal available tank volume of the aircraft 1. Optionally, that tank 52may be arranged to be used exclusively for ground-based operations ofthe aircraft 1. A selection between the other tanks 50, 53 based oncalorific value may then be made in flight based on engine thrust.

In implementations such as that shown in FIG. 14 , in which a trim tank52 is present on the aircraft 1 and is used as the first fuel tank 52,fuel would be drawn from the trim tank 52, which therefore decreases inmass due to the loss of fuel, during ground-based operations, forexample on-stand operations, warm-up, and optionally during one or moreof taxi, and take-off roll. In particular, if the trim tank 52 is filledwith SAF (or a high %SAF mixed fuel), the air quality advantagesobtained by using that SAF from the trim tank 52 for the APU 44 while onthe stand may be obtained, and that SAF may additionally be used for themain gas turbine engines 10 during warm-up, taxi, take-off, and possiblysome of climb (optionally as a mix with another fuel), until thetrim-tank 52 is empty. In some cases, the amount of fuel in the trimtank 52 may be selected such that the trim tank 52 is at leastsubstantially empty by take-off, so that it may get (partially) refilledduring climb, so as to enable its use for trimming the aircraft 1 evenduring climb. In other examples, it may be used for trimming theaircraft 1 only later in the flight. Fuel from a different fuel source50, 53 may be used thereafter. Such implementations may be of particularutility when the aircraft 1 is conducting missions towards the limit ofits payload-range capability; allowing all fuel tanks to be filled totheir fullest capacity initially whilst still providing trim tankcapacity promptly after take-off / before the trim tank 52 is usuallyused.

As such, by the time the aircraft 1 gets airborne, the first fuel tank52 would be relatively light, if not completely empty, and would notaffect centre of mass significantly. The first fuel tank 52 is thereforeavailable for normal use as a trim tank 52 for at least the cruise partof the flight - fuel from one or more of the other fuel sources 50, 53can be pumped into it when ready to reduce drag during cruise, andoptionally also during climb (after at least substantially emptying thefirst fuel tank 52 at the latest part way into climb).

The amount of fuel initially in the trim tank 52 may be selected toenable that fuel to be fully used up well before the aircraft 1 reachesits cruise altitude. More particularly, in such implementations, theamount of fuel supplied to the first fuel tank 52 on filling may becalculated so as to be at least substantially used up by the time theaircraft 1 takes off, and optionally more specifically by the time theaircraft rotates (rotation being what happens towards the end of thetake-off roll when the nose wheel of the aircraft 1 leaves the ground,but the main landing gear is still on the ground. In the momentsfollowing rotation, the aircraft 1 gains speed and then the main landinggear also leaves the ground). The fuel in the first fuel tank 52 of suchexamples may be selected based on its air quality and pollution effects,and may or may not have a lower calorific value than the fuel(s) inother tanks 50, 53.

In examples in which the first fuel tank 52 is used as a trim tank 52,an onboard fuel manager 214 may be arranged to detect the fuel level inthe first fuel tank 52 and automatically switch supply to a differenttank (irrespective of how much additional nvPM may be caused) if itdetects imminent running-dry of that tank 52, to avoid any fuel supplyinterruption to the engine 10, 44. It will be appreciated that having arelatively small amount of fuel remaining in the trim tank 52 would notprevent it from being used as such, and/or that the fuel manager 214 maybe arranged to pump fuel out of the trim tank 52 and into a differenttank before the take-off run commences, optionally following a negativeresult to a check that the trim tank 52 is sufficiently empty for thefore/aft centre of gravity to be within acceptable limits.

In some implementations, the fuel manager 214 may be arranged toautomatically switch from the first fuel tank 52 to another, optionallylarger, tank 50, 53 before commencing the take-off roll, to eliminatethe possibility of the first tank 52 running dry during the take-offroll and/or climb-out.

In some implementations, the fuel manager 214 may be arranged to requestor enforce more engine idle time before commencing a take-off run toensure that the trim tank 52 is sufficiently empty for use, if trimmingof the aircraft 1 is expected to be needed during the take-off runand/or if no spare capacity is available for fuel to be pumped out ofthe trim tank 52.

In examples in which detection is used to determine one or more fuelcharacteristics, the detection may be performed in each tank 50, 52, 53(or in one tank of each fuel source), and fuel characteristics for aresultant fuel blend from different tanks may then be calculated whereappropriate based on blend ratios. Alternatively or additionally, thedetection may be performed on approach to the engine 10, e.g. in apipe/fuel line which may contain a blended mix from multiple tanks.

Where data are collected for a fuel blend, e.g. on approach to theengine 10, data may be recorded as the blend is changed (by taking moreor less fuel from a given tank 50, 52) so as to determine thecomposition of fuel in each tank 50, 52, 53 individually. Thisdetermination may allow fuel selection or blend to be tailoredin-flight, e.g. using different fuels as appropriate for different partsof a flight envelope, even when fuel composition is not known ontake-off. Further, in some implementations such a determination may beperformed during engine warm-up, and/or in the early stages of taxi, sothat fuel selection for the rest of the taxiing may be adjusted asappropriate, for example selecting a fuel or fuel blend with maximumnvPM benefits whilst still at the airport.

In some examples, detection may be performed on the fuel immediatelybefore entering the engine 10 / combustor 16, optionally to ensure thatthe correct fuel/fuel blend is identified (e.g. as a check of theintended composition if this is already known) and that the data on fuelbeing burned are as up-to-date as possible (near real-time).

Once the calorific value of each fuel available for supply to the gasturbine engine 10 has been determined, by any suitable method, a singlefuel (from a single tank) or fuel blend (from multiple tanks) may beselected and provided to the gas turbine engine 10, based on a thrustdemand of the gas turbine engine 10. In particular, a fuel (single fuelor blend) with a lower calorific value may be supplied to the gasturbine engine 10 at lower thrust demand. Correspondingly, a fuel(single fuel or blend) with a higher calorific value may be supplied tothe gas turbine engine 10 at higher thrust demand. The fuel controlbased on calorific value may be performed in flight only in somescenarios, allowing fuel supply for ground-based operations to becontrolled differently (e.g. prioritising reduction of nvPM generationover matching calorific value to power demand).

It will be appreciated that thrust demand may be determined using anyone or more approaches known in the art, for example based on fuel flowrate and/or power lever angle in the cockpit, or one or more other pilotsettings, and optionally taking account of outside air density, or aproxy for it such as altitude, ambient temperature, and/or pressure. Useof fuel flow rate alone may be insufficient due to differences in fuelflow ranges at altitude as compared to on the ground.

The variation in calorific value of the fuel corresponding to thrustdemand may facilitate maintenance of a more constant fuel flow rate,and/or more even fuel pump and spill operation in flight. In general,the fuel mass flow rate varies widely between different parts of theflight for an aircraft 1, so differences in fuel calorific value are notbig enough to compensate for differences in fuel energy flowrequirements between climb and initial cruise, for example. However,during a level cruise segment where the aircraft 1 is burning off fuelat a constant altitude, the thrust requirement may decline slowly over asustained period, and fuel flow calorific value adjustments may allow anapproximately constant flow to be maintained. A substantially constantfuel flow rate may therefore be maintained in certain portions ofaircraft operation by implementing the approach disclosed herein.Further, maxima in fuel flow rate may be reduced, and/or minima in fuelflow rate may be increased, by choosing a fuel with a suitable calorificvalue at the corresponding points of aircraft operation. The “moreconstant fuel flow rate” may therefore refer to a decrease in themaximum spread of fuel flow rates across an entire engine operationenvelope / flight envelope.

It will be appreciated that pump speed is generally linked to shaftspeed in some aircraft 1, with a spill ratio being adjusted as requiredfor a given speed, such that fuel flow rate to the combustor 16 iscontrolled by a fuel delivery system 220 (e.g. comprising ahydro-mechanical unit, HMU) comprising multiple components, rather thanby a fuel pump alone. This more even operation may be beneficial interms of providing a suitable flow of fuel through the system, forexample for lubrication, for fueldraulics, and for heat transfer, evenat very low power demands. Use of a lower calorific value fuel in theengine 10 at lower power demands can assist with heat management, as thehigher flow rate of fuel passing through the engine provides more of aheat transfer medium. Further, it can be difficult to run large engines10 smoothly at low idle thrust using standard fuels - being able toswitch to a fuel with a lower calorific value could therefore improveperformance at low idle thrust.

A method 2060 of operating an aircraft 1 comprising a gas turbine engine10 and a plurality of fuel tanks 50, 52, 53 arranged to store fuel topower the gas turbine engine 10 is shown in FIG. 11 .

The method 2060 comprises arranging 2062 each fuel source/tank 50, 52 ofthe plurality of fuel tanks to contain a different fuel to be used topower the gas turbine engine 10, wherein the fuels have differentcalorific values. In some examples, one or more of the fuel tanks 50, 52may be part of a separate set of interlinked fuel tanks. In otherexamples, each fuel tank 50, 52 may be a stand-alone, single-tank, fuelsource.

The method 2060 further comprises storing 2064 information on the fuelcontained in each fuel tank 50, 52, optionally in memory of an on-boardfuel manager 214.

The method 2060 further comprises controlling 2066 the fuel input to thegas turbine engine 10 by selection of a specific fuel or fuelcombination from one or more of the plurality of fuel tanks 50, 52. Thecontrol 2066 may be performed only in flight, or throughout aircraftoperation. The selection is made based on thrust demand of the gasturbine engine 10 such that a fuel with a lower calorific value issupplied to the gas turbine engine 10 at lower thrust demand (e.g.ground operations (if implemented on the ground), descent, cruise), anda fuel with a higher calorific value is supplied to the gas turbineengine 10 at higher thrust demand (e.g. climb).

Even for an example with only two separate fuel sources 50, 52, a rangeof different fuel calorific values may be provided by dynamicallyblending the two fuels to different levels, depending on thrust demand.In some arrangements, the method 2060 comprises switching between two,three, four, or five fuels, and/or pre-set blends, with determinedcalorific values, depending on thrust demand. In other arrangements, theblend may be changed as a function of thrust demand, optionallycontinuously (within limits of precision of the fuel pump and/or otherflow controllers).

Additional data may be used in conjunction with the determined fuelcharacteristics to adjust control of the propulsion system 2. Forexample, the method may comprise receiving data of current conditionsaround the aircraft 1 (either from a provider, such as a third-partyweather-monitoring company, or from on-board detectors). These receiveddata (e.g. weather data, temperature, humidity, presence of a contrail,etc.) may be used to make or influence changes in the composition offuel supplied to the gas turbine engine 10. Instead of, or as well as,using “live” or near-live weather data, forecast weather data for theaircraft’s route may also be used to estimate current conditions.

A propulsion system 2 for an aircraft 1 may therefore comprise a fuelmanager 214 arranged to store information on the fuel contained in eachfuel tank 50, 52 and to control fuel input to the main gas turbineengine(s) 10, and optionally also to an APU 44, in operation. The fuelmanager 214 may be provided as part of a fuel delivery system 220arranged to allow control and adjustment of the fuel supplied to the gasturbine engine 10.

The fuel delivery system 220, as shown in FIG. 12 , may comprise one ormore flow controllers 216, such as valves and pumps, arranged to becontrolled by the fuel manager 214 so as to control fuel input to themain gas turbine engine(s) 10, and optionally also to an APU 44. Forexample, one flow controller 216 a, 216 b may be provided between eachfuel source and each engine 10. Such arrangements may allow differentfuels to be supplied to different engines 10 of the same aircraftpropulsion system 2.

Optionally, the fuel manager 214 may additionally receive other data (inaddition to fuel characteristic data), and use that other data and thefuel characteristic data to determine a desired fuel or fuel blend forthe gas turbine engine 10. The fuel manager 214 may be provided as aseparate fuel management unit 214 built into the propulsion system 2,and/or as software and/or hardware incorporated into the pre-existingaircraft control systems (e.g. into the EEC 42). In some examples, thefuel composition data may be stored separately from the circuitryperforming the fuel supply management and retrieved when required -wherever the data are stored, that storage can be thought of as a partof the fuel manager 214, whether or not it is integral therewith orindeed physically connected thereto in any way.

The broader term “power system” 4 may be used for the propulsion system2 to ensure that implementations in which the fuel is additionally oralternatively supplied to an APU 44 are captured, as propulsive powermay not always be provided by such power systems 4, e.g. whileperforming at-gate operations whilst the aircraft 1 is stationary (itwill also be appreciated that the main gas turbine engines 10 can alsobe used to provide non-propulsive power in many implementations).

The fuel manager 214 may be arranged to select a specific fuel or fuelcombination from one or more of the plurality of fuel tanks 50, 52, 53based on thrust demand of the gas turbine engine 10. In particular, afuel with a lower calorific value is supplied to the gas turbine engine10 at lower thrust demand, and vice versa. A fuel with a highercalorific value may therefore be used at high-power stages of the flightenvelope, such as during take-off. Calorific value of the fuel may beadjusted linearly with a % increase or decrease in thrust demand in somescenarios, within an available range.

The fuel manager 214 may be arranged such that a fuel with a lowercalorific value is supplied to the gas turbine engine 10 at cruise ascompared to that supplied during climb. Optionally, a fuel with a stilllower calorific value may be supplied to the gas turbine engine 10 atground idle or low idle - this same fuel may also be supplied to the APU44 in some implementations.

It will be appreciated that the term “low idle” is a generic termgenerally used for the idle setting for either ground or flight idlewhen the engine is operating to one of its minimum limiters (e.g.minimum speed, pressure, and/or temperature limits), set within the EEC42, with the throttle lever position being in the idle detent position.

The idle power level in flight can vary significantly depending onfactors such as altitude, power offtake, customer bleed, and anti-icingdemands; the term “low idle” therefore covers a range of power/thrustdemands.

Idle operation whilst the aircraft 1 is operating on the ground isreferred to as ground idle’ and idle operation whilst the aircraft 1 isoperating in flight is referred to ‘flight idle’.

High idle is a more specific term, referring to conditions where theaircraft 1 is in an approach landing configuration and idle is raisedabove flight low idle for the purposes of achieving adequate thrustresponse if required for a go-around. Whilst the throttle remains in theidle detent condition, there is an increased thrust level, and high idlecan only apply in flight (not for operations on the ground). In someimplementations, a fuel with a higher calorific value may be supplied tothe gas turbine engine 10 at high idle than at low idle, and a fuel witha still higher calorific value may be supplied when the thrust demandrises above that for high idle.

The fuel manager 214 may make changes to the fuel supply directly, ormay provide a suggestion or notification to the pilot regarding thechange, for approval (e.g. as described above for the propulsion systemcontroller 42, noting that the fuel manager 214 may be a part of, or incommunication with, the propulsion system controller 42). In someexamples, the same fuel manager 214 may automatically make some changes,and request others, depending on the nature of the change.

In some examples, an aircraft 1 may be modified to perform the method2060 described above, optionally by installing an adjustable fueldelivery system 220.

A method 2070 of modifying an aircraft 1 in such a way is shown in FIG.13 . The original aircraft 1 comprises a gas turbine engine 10, which,in the example being described, comprises an engine core 11 comprising aturbine 19, a compressor 14, and a core shaft 26 connecting the turbineto the compressor. The aircraft 1 also comprises a plurality of fueltanks 50, 52 and a fan 23 located upstream of the engine core, the fancomprising a plurality of fan blades and being arranged to be driven byan output from the core shaft. The aircraft 1 may also comprise an APU44.

The method 2070 comprises arranging 2072 each fuel tank 50, 52, 53 (orat least two fuel tanks of a plurality of fuel tanks) to contain adifferent fuel to be used to power the gas turbine engine(s) 10, 44,wherein the fuels have different calorific values.

In some cases, the aircraft 1 may already comprise a plurality of fueltanks 50, 52, 53 arranged to store fuel to power the gas turbine engine10; in such examples, the step 2072 of arranging the fuel tanks maysimply comprise filling the tanks selectively with different fuels. Incases in which the aircraft 1 previously only had a single fuel tank 50,a new fuel tank 52 may be added so as to provide a plurality of fueltanks. In cases in which the aircraft 1 previously only had a singlefuel source, albeit comprised of multiple tanks, a new fuel tank 52 maybe added and/or fuel lines may be adjusted such that the original tanks50, 53 are no longer all fluidly interconnected, so providing at leasttwo separate fuel sources. The arranging step 2072 may therefore varydepending on the initial aircraft configuration.

The method 2070 further comprises providing 2074 a fuel manager 214arranged to store information on the fuel contained in each fuel tank50, 52, 53 and to control fuel input to the gas turbine engine 10. Thefuel manager 214 may operate only in flight. In examples in which itoperates both in flight and during ground-based operations, the controlstrategy it employs may differ between flight and ground-basedoperations in some examples. In other examples, for example where allavailable fuels have acceptably low nvPM emissions, fuel control basedon thrust and calorific value may additionally be performed duringground-based operations.

The storage and control functions may be performed by separate entitiesor the same entity; it will be appreciated that the fuel manager 214 maytherefore be a distributed system or a single unit or module. The stepof providing 2074 the fuel manager 214 may comprise or consist ofinstalling software in extant memory, to be executed using extantsystems, in some examples. In other examples, a new physical unit ormodule may be mounted onto the propulsion system 2, optionally includingone or more flow controllers 216 and/or replacement fuel line sectionsas appropriate to achieve the desired fuel flow and mixing control.

The fuel manager 214 is arranged to control fuel input to the gasturbine engine 10 by selection of a specific fuel or fuel combinationfrom one or more of the plurality of fuel tanks 50, 52, 53 based onthrust demand of the gas turbine engine 10 such that a fuel with a lowercalorific value is supplied to the gas turbine engine 10 at lower thrustdemand, and vice versa. This control may be performed throughoutaircraft operation, or only at certain stages (e.g. only in flight, oronly during cruise).

The inventors also appreciated that current standards mean that (pure)SAF cannot be used in commercial flight, but SAF could be used forground-based operations, for example to reduce airport emissions.Similarly, there may be advantages to using such a sustainable aviationfuel for ground-based operations even when it can also be used inflight, for example to maximise environmental benefits from a limitedavailable amount of SAF. Further, if fuels which are not pure SAF butinstead have a proportion of SAF are available to the aircraft 1, usingthe fuel(s) with the highest %SAF for ground-based operations maycorrespondingly reduce airport emissions and so improve air quality.Re-design of the aircraft’s fuel system may therefore allow technicaland environmental benefits of SAF to be realised, whether or not the SAFis provided as part of a blend.

If only a relatively small amount of pure SAF or a high-percentage SAFblend is available to an aircraft 1 (the rest of the fuel being eitherpetroleum-based fuel or a lower-percentage SAF blend), the most benefitfrom that SAF (or high-percentage SAF blend) may therefore be obtainedby using that fuel in and around the airport, where power demand isrelatively low (noting nvPM generation discussions above).

For the same reasons, this SAF or high %SAF blend, optionally stored ina first fuel tank 52 as described above, may be used in the auxiliarypower unit (APU) 44 of the aircraft 1, e.g. at the gate of an airport.

Whilst it will be appreciated that a synthetic fuel could be made toexactly mimic a traditional kerosene fuel, one or more fuelcharacteristics of SAF stored onboard the aircraft 1 may differ from thefuel characteristics of the one or more other fuels stored onboard theaircraft 1, in other tanks.

The fuel characteristics may include one or more of the fuelcharacteristics described above, and may be determined using any of theapproaches described above, including by any of the example detectiontechniques listed.

If two or more different SAFs, or two or more different SAF blends withthe same %SAF, are available to the aircraft 1, one or more other fuelcharacteristics – such as the hydrogen to carbon ratio (H/C) of the fuelor level of non-volatile particulate matter (nvPM) emissions oncombustion - may be used to select between the two or more fuels withthe same %SAF. One or more other parameters likely to influence airquality around an exhaust from the aircraft 1 may also be compared so asto select the fuel likely to provide the best air quality outcomes.Environmental factors (e.g. airport altitude and humidity) may also beconsidered in this assessment.

In the present examples, described with respect to FIGS. 14 and 18 , thefirst fuel source is the first fuel tank 52. In other examples, thefirst fuel source may comprise multiple interlinked tanks.

In the example being described, the first fuel tank 52 is arranged tocontain only a fuel which is a pure sustainable aviation fuel (SAF),i.e. 100% sustainably sourced and not kerosene derived / of fossilorigin. In other examples, multiple fuel tanks of a plurality of fueltanks may all contain SAF - any one of the subset of fuel tankscontaining SAF may therefore be used to supply SAF; it will beappreciated that the example of just one fuel tank 52 containing SAF isdescribed here by way of non-limiting example only.

In other examples, the first fuel tank 52 is arranged to contain a fuelwhich is a SAF blend with a higher %SAF than that in any other fuel tank50, 53. In other examples, multiple fuel tanks of a plurality of fueltanks may all contain a SAF blend with the same %SAF, such that thereare multiple first fuel tanks 52 – any one of the first fuel tanks maytherefore be used to supply fuel for at least the majority ofground-based operations; it will be appreciated that the example of justone first fuel tank 52 is described here by way of non-limiting exampleonly. One or more second fuel tanks 50, 53 contain one or more fuelswith a lower %SAF (optionally 0%SAF) and are used for other operations.

The example shown in FIG. 18 is generally similar to that shown in FIG.14 , but the differences are described below. (in FIG. 18 , one internalfuel line is shown – to the larger tank 50; it will be appreciated thatat least a second fuel line to the first fuel tank 52 would generallyalso be provided but that is not shown for clarity – similarly, no suchfuel lines are shown in FIG. 14 for clarity, but would generally bepresent.)

In the examples currently being described, the aircraft 1 has aplurality of fuel tanks 50, 52, 53, and in particular, at least two fuelsources / separate tanks 50, 52, and optionally more. Each fuel tank 50,52 is arranged to store a fuel to be used to power one or more gasturbine engines 10, 44 of the aircraft. One of the fuel tanks 52 –referred to as the first fuel tank 52 – is arranged to contain only afuel which is a sustainable aviation fuel (SAF), or to contain a high%SAF fuel blend. In some implementations, such as that shown in FIG. 18, this first fuel tank 52 is arranged to only ever contain the SAF orhigh %SAF blend, and may always be isolated from the other fuelsource(s) 50, 53. In other implementations, such as that shown in FIG.14 , this first fuel tank 52 is arranged to contain the SAF or high %SAFblend initially, and to be fluidly isolated from the other fuelsource(s) 50, 53 whilst that fuel is being used (e.g. for ground-basedoperations), but may then be fluidly connected to one or more other fuelsources 50, 53 in flight (e.g. by opening one or more valves), and mayhave a different fuel from a different fuel source 50, 53 pumped into it(e.g. to serve as a trim tank 52).

The first fuel tank 52 of such examples is therefore arranged to containonly a fuel which is a sustainable aviation fuel (SAF), or high %SAFblend, at least during ground-based operations. In implementation inwhich the fuel is 100% SAF, the SAF in that tank 52 may be selected suchthat the propulsion system 2 can be run on that fuel alone (for example,for ground operations, in case of an emergency in flight, or if fuelregulations change such that flight on 100% SAF is generally permitted),or may be tailored for use in an APU 44 only and not suitable forcombustion in a main gas turbine engine 10. The fuel tank 52 containingthe sustainable aviation fuel only may therefore be arranged to be usedto power the aircraft 1 when the aircraft is performing operations onthe ground. Optionally, the first fuel tank 52 may be used to supply SAFto the gas turbine engine 10, but optionally only during ground-basedoperations. Alternatively, the first fuel tank 52 may be used to supplySAF to the gas turbine engine 10 as part of a blend in flight, or may bearranged to supply SAF only to the APU 44.

In implementations in which the fuel in the first fuel tank(s) 52 is notpure SAF, more flexible use may be made of that fuel in flight evenunder current regulations at the time of writing.

In some implementations, the first fuel tank 52 may be used to supplyfuel to both the main gas turbine engine 10 and the APU 44.

In some of the examples being described, all fuel used for groundoperations is sustainable aviation fuel or the highest %SAF blendavailable, and all fuel used for ground operations is therefore takenfrom the first fuel tank 52 (in examples with multiple first tanks, anyone or more of those tanks may be used). In other examples, most of thefuel used for ground operations is SAF or the highest %SAF blendavailable, with only small amounts from other sources being used (e.g.less than 10% or less than 5% of the fuel use and/or of the operationtime, or using fuel from another source only for initial enginestart-up).

It will be appreciated that, if a first fuel tank 52 runs out of fuel, asecond fuel tank 50, 53 with the highest %SAF blend amongst the secondfuel tanks may be reclassified as a first fuel tank and used forremaining ground-based operations.

A fuel manager 214 may be arranged to control fuel input to the gasturbine engine(s) 10, 44 so as to take only fuel from the one or morefirst fuel tanks 52 when the aircraft 1 is performing at least themajority of operations on the ground. As used herein, “operations on theground” generally refers to operations prior to take-off, and mayinclude one or more of:

-   · Start-up of the engine itself;-   · Heating, lighting, air conditioning and/or other non-propulsive    demands, whilst the aircraft 1 is stationary (e.g. at a gate) or    moving;-   · Taxiing of the aircraft 1; and-   · Initiation of take-off roll, optionally including raising a nose    wheel, if appropriate.

The first fuel tank 52 (or another one or more fuel tanks containing SAFor the high %SAF blend, in other examples) may therefore be used toprovide some or all of the fuel used by the aircraft power system 4on-stand (e.g. at a gate), and during warm-up, taxi, and take-off roll.

Beneficially, this may meet airport requirements for emissions and/oruse of SAF.

If, once the operations on the ground are complete, there is still somefuel left in one or more of the first fuel tanks 52, optionally it couldbe reserved for use at the destination airport for further groundoperations on landing, including landing roll and taxi-in.

Any leftover fuel in the one or more first fuel tanks 52 mayadditionally or alternatively be used for the early part of climb (ontake-off) and/or the final part of approach (on landing) – i.e. for oneor more non-ground parts of the flight that are nonetheless close to theground and therefore relevant to local air quality and human health. Useof SAF on approach to a destination airport (as opposed to on take-off)may be of particular nvPM benefit, as the power requirement is lower andso the reduction in nvPM achievable by use of SAF higher.

A selection may be made based on the amount of SAF available, the SAFblend types available, and a priority order for SAF usage in the variousparts of the aircraft’s operations.

The fuel manager 214 may be arranged to control fuel input to thepropulsive gas turbine engine(s) 10 in flight by selection of a specificfuel or fuel combination from one or more of the plurality of fuel tanks50, 52.

The first fuel tank 52 of the plurality of fuel tanks 50, 52, arrangedto contain only the fuel with the highest %SAF (which may be SAF) may besmaller than the one or more other fuel tanks 50, 53. For example, thefirst fuel tank 52 may represent 3% to 20 %, and optionally 5% to 10%,of the total available tank volume of the aircraft 1. Optionally, thattank 52 may be arranged to be used exclusively for ground-basedoperations of the aircraft 1.

Each fuel tank 50, 52, 53 onboard the aircraft 1 may be arranged tocontain a fuel of a different type (e.g. petroleum-origin fuel or SAF,or different SAF varieties), and some tanks may contain blended fuelswith a proportion of a sustainable aviation fuel mixed with atraditional jet fuel or other petroleum-origin fuel. In some examples,two or more tanks 50, 52 may contain the same fuel, provided that atleast two different fuels are available from the fuel tanks on theaircraft 1 overall. In some examples, at least one tank 52 contains SAF-i.e. purely a sustainable aviation fuel, not a blend.

In implementations such as that shown in FIG. 14 , in which a trim tank52 is present on the aircraft 1 and is used as the first fuel tank 52,the approach and benefits relating to SAF use described above may alsobe obtained.

The propulsion system 2 of the examples being described again comprisesan adjustable fuel delivery system 220, allowing a selection to be madeof which tank(s) 50, 52, and therefore what fuel or fuel blend, to use.In such examples, the fuel characteristics may vary over the course of ajourney (including both flight and the ground-based operations at thebeginning and/or end of a journey) - a specific fuel or fuel blend maybe selected to improve operation at certain flight stages or in certainexternal conditions.

In examples in which detection is used for one or more fuelcharacteristics (either by direct detection, or by inference fromdetected parameters), e.g. to discover or verify which tank contains thehighest %SAF fuel, any of the detection approaches described above maybe implemented. In other examples, no detection may be performed andsupplied data on fuel composition may be relied upon instead – that datamay be simply SAF proportions, e.g. “100% SAF” vs. “Other”, or %SAF foreach tank, or may include more detailed fuel characteristic information.In other examples, no fuel data at all may be supplied – instead, eachtank 50, 52, 53 may be identified as a “SAF-only” tank or a“non-SAF-only” tank, or as a “Highest %SAF” tank or “Other” tank, andthe method may rely on the tanks 50, 52, 53 being correctly filledaccordingly.

In some examples, calorific values for each available fuel may becalculated or provided, and a fuel or fuel blend supplied based onthrust demand as described above (optionally also considering altitudein flight) – some of the fuel from the first tank 52 may be used aloneand/or in one or more blends in such examples. In examples with only onefirst tank 52, control based on calorific value may be performed inflight only. In examples with more than one first tank 52, a selectionbetween first tanks may be made based on calorific value for differentthrust requirements during ground-based operations, too.

A method 2061 of operating an aircraft 1 comprising a gas turbine engine10, 44 and a plurality of fuel tanks 50, 52 arranged to store fuel topower the gas turbine engine 10, 44 is shown in FIG. 15 .

The method 2061 comprises arranging 2063 at least two fuel tanks 50, 52of the plurality of fuel tanks to each store a different fuel, inparticular, fuels with different proportions of SAF. A first fuel tank52 contains fuel with a higher proportion of SAF than that in a secondfuel tank 50, 53. A first fuel tank 52 of the plurality of fuel tanksmay be arranged to contain only a fuel which is a sustainable aviationfuel.

This arranging step 2063 may comprise fluidly isolating one or moretanks from each other, if required, so as to allow different fuels to bestored in different tanks (e.g. by closing valves). This arranging step2063 may comprise filling the tanks 50, 52, 53 appropriately.

In some examples, one or more of the fuel tanks 50, 52 may be part of aseparate set of interlinked fuel tanks. In other examples, each fueltank 50, 52 may be a stand-alone, single-tank, fuel source.

In some examples, the fuel in the first fuel tank 52 has a %SAF greaterthan 50%, for example being greater than or equal to 55%, 60% or 70%,and optionally may be 100% SAF.

The method 2061 of some examples comprises identifying 2067 which tank52 contains the fuel with the highest proportion of a sustainableaviation fuel. If several tanks 50, 52 each contain fuel with the same,highest, %SAF, the several tanks may all be identified as first fueltanks 52. Optionally, one or more other characteristics (e.g. calorificvalue, nvPM emissions) may be used to select which first fuel tank 52 touse in such scenarios. The identification 2067 may be performed bydetection or determination from engine operating parameters, for exampleusing any of the approaches described herein, or using supplied data(e.g. data transmitted to or otherwise obtained by or accessed by thefuel manager 214).

The method 2061 further comprises controlling 2065 fuel input to the gasturbine engine 10 so as to use only fuel from the tank 52 containing thefuel with the highest proportion of a sustainable aviation fuel (or, insome cases, any fuel with more than 50% SAF) when the aircraft 1 isperforming at least the majority of operations on the ground. That fuelmay have a SAF proportion of more than 50% SAF, and optionally at least55% SAF. Here, using that fuel for “at least the majority” of operationsmay mean that at least 90% or 95% of the fuel used for ground-basedoperations is that fuel, and/or at least 90% or 95% of the operationtime for ground-based operations is powered by that fuel, and/or thatthe fuel with the highest proportion of SAF is used for all ground-basedoperations except initial engine start-up (for which a dedicated fuelmay be used as described below).

The method 2061 optionally further comprises storing 2064 information onthe fuel contained in each fuel tank 50, 52, optionally in memory of anon-board fuel manager 214. The information stored may simply be a flagas to whether or not a particular tank 50, 52 contains the fuel with thehighest %SAF, or a fuel with over 50% SAF. Additional information may bestored in other examples. This stored information may be used for thecontrolling step 2065, and in particular may be used to identify thefirst fuel tank 52 (and/or correspondingly one or more other tanks, ifmultiple tanks contain SAF or the fuel with the highest %SAF), if thatis not hard-coded / hard-wired into the propulsion system 2. A look-uptable of tank properties may be used to identify the first fuel tank(s)52 instead if the tanks are always arranged to contain specific fuels.

A power system 4 for an aircraft 1 may therefore comprise a fuel manager214 arranged to store information on each fuel tank 50, 52 / on the fuelcontained in each fuel tank 50, 52 and to control fuel input to the gasturbine engine 10 in operation. The information stored may simplycomprise a flag for whether or not each tank is the highest %SAF tank,or may comprise more detailed information, such as a %SAF content foreach tank, and/or one or more other fuel characteristics of the fuelcurrently in each tank 50, 52 (%SAF may be defined by volume and/ormass, noting that densities can vary within accepted bounds). In suchexamples, which tank 52 of the plurality of fuel tanks 50, 52, 53 is thefirst tank 52 may vary over the lifetime of the power system 4, forexample depending on which tank is filled with which fuel. In otherexamples, the fuel delivery system 220 as shown in FIG. 16 may be set upfor one specific tank 52 to always be the highest %SAF tank, and no suchfuel information may need to be stored.

The fuel manager 214 may also be arranged to identify which tank 52contains the fuel with the highest proportion of a sustainable aviationfuel, and/or to identify all tanks containing fuels with greater than50% SAF.

In implementations in which the SAF or highest %SAF blend is supplied toa main gas turbine engine 10 of the aircraft 1, which providespropulsive power to the aircraft 1, the power system 4 may be referredto more specifically as a propulsion system 2. The broader term “powersystem” 4 mentioned above is used here to ensure that implementations inwhich the fuel is additionally or alternatively supplied to an APU 44are captured, as propulsive power may not be provided by such powersystems 4.

The fuel manager 214 may be provided as part of a fuel delivery system220 arranged to allow control and adjustment of the fuel supplied to thegas turbine engine 10, 44, and may be as described above for the earlierexamples.

The fuel manager 214 is generally as described above. In examples withAPUs 44, the fuel manager 214 may additionally be arranged to controlthe fuel or fuel blend provided to the APU 44.

In some of the examples being described, the fuel manager 214 isarranged to take fuel exclusively from the first fuel tank 52, i.e. thefuel tank 52 containing the fuel with the highest proportion of SAF, forground-based operations of the power system 4.

As described above for other examples, the fuel manager 214 mayadditionally receive other data (in addition to information denotingwhich tank(s) contain(s) SAF, providing a %SAF for each tank, and/orproviding other fuel characteristic data), and use that other data andthe fuel characteristic data to determine a desired fuel or fuel blendfor the gas turbine engine 10, 44 in flight.

The fuel manager 214 may be arranged to control fuel input to the gasturbine engine 10, 44 so as to take fuel primarily, or only, from afirst fuel tank 52 when the aircraft 1 is performing operations on theground.

In some examples, an aircraft 1 may be modified to perform the method2061 described above, optionally by installing an adjustable fueldelivery system 220.

A method 2071 of modifying an aircraft 1 in such a way is shown in FIG.17 . The original aircraft 1 comprises a gas turbine engine 10, the gasturbine engine 10 of this example comprising an engine core 11comprising a turbine 19, a compressor 14, and a core shaft 26 connectingthe turbine to the compressor. The aircraft 1 also comprises a fan 23located upstream of the engine core, the fan comprising a plurality offan blades and being arranged to be driven by an output from the coreshaft. The original aircraft 1 may further comprise an APU 44, the APUitself being or comprising a gas turbine engine 44.

The method 2071 comprises arranging 2073 at least two separate fueltanks 50, 52 to store a different fuel, such that one fuel has a higherproportion of SAF than the other. One or more fuel tanks may be arrangedto contain only a fuel which is a sustainable aviation fuel in someimplementations (in an example being described, the first fuel tank 52of the plurality of fuel tanks 50, 52 is the only pure SAF-containingtank).

In some cases, the aircraft 1 may already comprise a plurality of fueltanks 50, 52 arranged to store fuel to power the gas turbine engine(s)10, 44; in such examples, the step 2073 of arranging the fuel tanks maysimply comprise filling the tanks selectively with different fuels. Incases in which the aircraft 1 previously only had a single fuel tank 50,a new fuel tank 52 may be added so as to provide a plurality of fueltanks. In cases in which the aircraft 1 previously only had a singlefuel source, albeit comprised of multiple tanks, a new fuel tank 52 maybe added and/or fuel lines may be adjusted such that the original tanks50, 53 are no longer all fluidly interconnected, so providing at leasttwo separate fuel sources. The arranging step 2073 may therefore varydepending on the initial aircraft configuration.

The method 2071 further comprises providing 2075 a fuel manager 214arranged to control fuel input to the gas turbine engine(s) 10, 44 so asto use only, or primarily, SAF when the aircraft 1 is performingoperations on the ground.

The fuel manager 214 may additionally be arranged to identify which tank52 contains the fuel with the highest proportion of a sustainableaviation fuel – this tank 52 may be identified as a first fuel tank 52.In cases in which two or more tanks contain a fuel with the same,highest, %SAF, a plurality of tanks may be identified as first fueltanks. A selection between these may be made using one or more of theapproaches described above.

The fuel manager 214 may use fuel from the first fuel tank 52 (e.g. SAFin a specific example described, or more generally the highest %SAF fuelavailable, or optionally any fuel with more than 50% SAF) for some, butnot all, ground-based operations. For example, the fuel manager 214 mayuse a different fuel for start-up before switching to using the firsttank 52, may switch to a different fuel if the fuel from the first fueltank 52 is used up before the operations on the ground are complete,and/or the fuel manager 214 may supply the fuel from the first fuel tank52 to the APU 44 but a different fuel to the other gas turbine engine(s)10.

In some examples, such as arrangements in which the tank(s) 52 used tostore the fuel with the highest %SAF may vary over the lifetime of thepropulsion system 2, the fuel manager 214 may additionally be arrangedto store information on the fuel contained in each fuel tank 50, 52 soas to allow the first tank 52, or equivalent(s), to be identified.

The storage and control functions may be performed by separate entitiesor the same entity; it will be appreciated that the fuel manager 214 maytherefore be a distributed system or a single unit or module. The stepof providing 2075 the fuel manager 214 may comprise or consist ofinstalling software in extant memory, to be executed using extantsystems, in some examples. In other examples, a new physical unit ormodule may be mounted onto the propulsion system 2, optionally includingone or more flow controllers 216 and/or replacement fuel line sectionsas appropriate to achieve the desired fuel flow and mixing control.

In some examples, the fuel manager 214 may be additionally arranged toperform other functions, for example to control fuel input to the gasturbine engine 10 by selection of a specific fuel or fuel combinationfrom one or more of the plurality of fuel tanks 50, 52 based on thrustdemand of the gas turbine engine 10 such that a fuel with a lowercalorific value is supplied to the gas turbine engine 10 at lower thrustdemand, and vice versa. It will be appreciated that thrust demand may bedetermined using any one or more approaches known in the art, forexample as mentioned above.

The inventors also appreciated that fuel differences may allow re-designof the aircraft’s fuel system to provide technical and environmentalbenefits for ground-based operations of an aircraft. In the presentexamples, described with respect to FIGS. 14 and 19 , usage of the firstfuel tank 52 is adjusted accordingly.

FIG. 19 shows an aircraft 1 with a propulsion system 2 generally as forthat shown in FIG. 14 , but with three separate fuel sources 50, 52, 53as opposed to two. First, second and third fuel sources are thereforenot fluidly coupled to each other so as to separate the different fuels(at least under normal running conditions). In other examples, such asthat shown in FIG. 14 , only two separate fuel sources may be provided.

In the present example, the first fuel source is the first fuel tank 52.In other examples, the first fuel source may comprise multipleinterlinked tanks. The first fuel tank 52 is arranged to be used atleast primarily (and in some cases, only) for ground-based operations.Fuel from the first fuel tank 52 is arranged to be used for at least themajority of ground-based operations, in a similar way to the usage ofSAF or the high %SAF blend described with respect to the precedingexamples.

In the example shown in FIG. 19 , the second fuel source comprises acentre fuel tank 50, located primarily in the fuselage of the aircraft,and the third fuel source comprises a plurality of wing fuel tanks 53 a,53 b, where at least one wing fuel tank is located in the port wing andat least one wing fuel tank is located in the starboard wing forbalancing. Tanks 53 a and 53 b are fluidly interconnected in the exampleshown, so forming a single, third, fuel source. Fluid interconnectionbetween the wing fuel tanks 53 of the third fuel source may be providedfor balancing of the aircraft 1, as described earlier.

Each of the centre fuel tank 50 and the wing fuel tanks 53 may comprisea plurality of fluidly interconnected fuel tanks.

In another example, the wing fuel tanks 53 a, 53 b may be fluidlyconnected to the central tank 50, so forming a single, second, fuelsource. For balancing purposes, one or more fuel tanks in the port wingmay be fluidly connected to one or more fuel tanks in the starboardwing. This may be done either via a centre fuel tank 50 (if that tankdoes not form part of a separate fuel source), or bypassing the centrefuel tank(s), or both (for maximum flexibility and safety).

In some examples, the allocation of fuel tanks 50, 52, 53 available onthe aircraft 1 may be constrained such that each fuel source issubstantially symmetrical with respect to the aircraft centre line. Incases where an asymmetric fuel tank allocation is permitted, a suitablemeans of fuel transfer may be provided between fuel tanks of the firstfuel source and/or between fuel tanks of the second fuel source suchthat the position of the aircraft’s centre of mass can be maintainedwithin acceptable lateral limits throughout the flight. However, inexamples in which the first fuel tank 52 is much smaller than the otherfuel tanks 50, 53, its change in mass as fuel from that tank is used maybe less significant and so symmetry may not be a concern.

In the examples shown in FIGS. 14 and 19 , the first fuel tank 52 isagain smaller than the second fuel tank 50. In the example of FIG. 14 ,it is located further towards the rear of the fuselage. The first fueltank 52 of this implementation may therefore be used more easily as atrim tank 52 in flight; the approach described above with respect totrimming may therefore also be used in conjunction with the initialdedication of a tank 52 for ground use. If the first tank 52 is to beused as the trim tank in flight, the fuel in that tank 52 is either usedup in the initial stages of ground operations and optionally take-off,or any remainder is mixed with other fuel pumped into that tank fortrimming in flight. By contrast, in implementations in which that firsttank 52 is not used as a trim tank, rather than being used up in flightor diluted, the remaining SAF in the first tank 52 could be kept untilthe aircraft 1 has landed and then used to power the final groundoperations (e.g. landing and/or taxi-in) to obtain further air-qualitybenefits at the destination airport.

In various examples, an APU 44 as described above may be used to providesome or all of the power for ground-based operations, and the first fueltank 52 may be used to provide fuel to the APU 44.

In alternative implementations, the first fuel tank 52 may be adedicated APU fuel tank, and may not be fluidly interconnected to themain gas turbine engines 10 as it is in FIG. 14 , nor to any other fueltank. In some such examples, a fuel manager as described elsewhereherein may not be present or used.

An aircraft 1 may be refuelled by connecting a fuel storage vessel 60,such as that provided by an airport fuel truck, or a pipeline, to a fuelline connection port 62 of the aircraft 1, via a fuel line 61, asdescribed above. In particular, in the examples being described, thefirst tank 52 can be fuelled directly from a fuel supply rather thanhaving to be filled by transfer from another fuel tank 50, 53 of theaircraft 1. In FIG. 14 , internal fuel lines from the port(s) 62 to thetanks are not shown, for clarity. In FIG. 19 , one internal fuel line isshown (to the larger tank 50); it will be appreciated that at least asecond fuel line to the first fuel tank 52 may also be provided – ratherthan filling the first tank 52 via a different tank – but that is notshown for clarity.

In examples of the present invention, the aircraft 1 has a plurality offuel tanks 50, 52, 53, and in particular, a first fuel tank 52 arrangedto be used to power some or all of the ground-based operation of theaircraft 1, and one or more secondary fuel tanks 50, 53, each arrangedto contain a fuel to be used to power the gas turbine engine 10 inflight. Ground-based operation of the aircraft 1 may involve one or moregas turbines 10, 44 – the gas turbine engine 44 of the APU 44 may beused for some ground-based operations, and one or more of the main gasturbine engines 10 may be used for other ground-based operations.

Each fuel tank 50, 52, 53 is arranged to store a fuel to be used topower one or more gas turbine engines 10, 44 of the aircraft 1. One ofthe fuel tanks – referred to as the first fuel tank 52 – is arranged tobe used for some or all ground-based operations of the aircraft 1.

In the example shown in FIG. 21 , the first fuel tank 52 is dedicated toground-based operations exclusively and fuel from that tank 52 is notused in flight. The fuel manager 214 of this example (which maygenerally be as described above) is arranged to control fuel input tothe gas turbine engines 10, 44 so as to take fuel from only the firstfuel tank 52 when the aircraft 1 is performing ground-based operations,and to take fuel from only the one or more secondary fuel tanks 50, 53for other operations. The flow controller 216 c may prevent fuel frombeing taken from the first fuel tank 52 in flight. In other examples,some fuel from the first tank 52 may also be used in flight, and/or somefuel from other tanks may be used for ground-based operations.

In some examples, the first fuel tank 52 is arranged to contain a fuelwhich is a sustainable aviation fuel (SAF), at least during ground-basedoperations – i.e. that tank 52 may contain 100% pure SAF. The fuel inthe first tank 52 may be selected such that the propulsion system 2 canbe run on that fuel alone for ground-based operations, whether or notthat fuel would be certified for use in flight. Alternatively, the fuelin the first tank 52 may not be suitable for use in the main gas turbineengines 10, and may be used solely for the APU 44.

In the examples currently being described, the first fuel tank 52 istherefore arranged to be used to power the aircraft 1 when the aircraftis performing operations on the ground, whether or not that fuel tank 52contains SAF.

In some implementations, such as that shown in FIG. 19 , this first fueltank 52 is arranged to only ever be used for ground-based operations(optionally with any small remainder after rotation of the aircraft 1being finished off during climb as described earlier), and may always beisolated from the other fuel source(s) 50, 53. In other implementations,such as that shown in FIG. 14 , this first fuel tank 52 is arranged tobe used for ground-based operations initially, and to be fluidlyisolated from the other fuel source(s) 50, 53 whilst it is being usedfor ground-based operations, but may then be fluidly connected to one ormore other fuel sources 50, 53 in flight, and may be used as a trim tank52, and/or to supply fuel to an engine 10, 44, in flight. The first fueltank 52 may have a different fuel from a different fuel source 50, 53pumped into it in flight, as compared to the fuel with which it wasoriginally filled for powering the ground-based operations.

In some examples, the first fuel tank 52 of some implementations isdesigned to be used to supply fuel to one or more gas turbine engines10, 44 only during ground-based operations, i.e. that tank is arrangedto be used exclusively for ground-based operations of the aircraft 1. Insome implementations, a fuel tailored for use in an APU 44 only and notsuitable for combustion in a main gas turbine engine 10 may be providedin the first tank 52. However, the fuel manager 214 of some examples maybe arranged to allow fuel from the first fuel tank 52 to be supplied tothe main gas turbine engine 10, for example alone in emergencysituations or as part of a blend in normal flight, or the first fueltank 52 may play a different role – such as that of trim tank – inflight. In other examples, a block may be provided (e.g. by a flowcontroller 216 c) to prevent any introduction of fuel from the firsttank 52 to the gas turbine engine 10 in flight.

In some of the examples being described, all fuel used for groundoperations is taken from the first fuel tank 52. The fuel manager 214may be arranged to control fuel input to the gas turbine engine(s) 10,44 so as to take fuel only from the first fuel tank 52 when the aircraft1 is performing all operations on the ground.

In other examples, the fuel taken from the first fuel tank 52 may beused for a subset of ground-based operations, with fuel from other tanks50, 53 being used where appropriate. For example, the first fuel tank 52may provide fuel to the APU 44 for its ground-based operations (e.g.lighting, air conditioning, main engine start), but a different fueltank 50, 53 may provide fuel to the main gas turbine engine(s) 10 forits ground-based operations (e.g. taxi), or a different fuel may be usedfor initial engine warm-up/start-up, and the supply may then be switchedto the first fuel tank 52 (e.g. once a threshold fuel temperature hasbeen reached). As used herein, “operations on the ground” are as definedabove.

The first fuel tank 52 may therefore be used to provide some or all ofthe fuel used by the aircraft power system 4 on-stand (e.g. at a gate),and during warm-up, taxi, and take-off roll.

Beneficially, this may allow a fuel tailored to the relativelylow-powered requirements of ground-based operations to be stored andused, and/or may facilitate meeting airport requirements for emissionsand/or use of SAF, and/or may improve functioning of the engine 10 atlow thrusts.

The first fuel tank 52 of the plurality of fuel tanks may be arranged tocontain only a fuel which is a sustainable aviation fuel, or to containa high %SAF blend; some or all of the nvPM and air quality benefitsdescribed above may therefore be provided. For the same reasons, 100% orhigh %SAF / fuel from the first fuel tank 52 may be selected to be usedin an auxiliary power unit (APU) 44 of the aircraft 1 at the gate of anairport.

The first fuel tank 52 of the plurality of fuel tanks may be smallerthan the one or more other fuel tanks. For example, the first fuel tank52 may represent 3% to 20 %, and optionally 5% to 10%, of the totalavailable tank volume of the aircraft 1.

In implementations such as that shown in FIG. 14 in which the first fueltank 52 may operate as a trim tank 52, the approach described above maybe used, whether or not the fuel in the first fuel tank 52 is SAF or ahigh %SAF blend.

Each fuel tank 50, 52, 53 onboard the aircraft may be arranged tocontain a fuel with a different type or proportion of a sustainableaviation fuel. Whilst it will be appreciated that a synthetic fuel couldbe made to exactly mimic a traditional kerosene fuel, one or more fuelcharacteristics of SAF fuel stored onboard the aircraft 1, either as apure SAF fuel or as part of a blend, may differ from the fuelcharacteristics of the one or more other fuels (SAF blends or otherwise)stored onboard the aircraft 1, in other tanks.

The fuel characteristics may include one or more of those listed above,and may be determined using any of the techniques listed above, alone orin combination as appropriate, including the various example detectionmethods mentioned.

In other examples, no detection may be performed and supplied data onfuel composition may be relied upon instead – that data may be simply“ground-use fuel” vs. “flight-use fuel”, or may include more detailedfuel characteristic information. In other examples, no fuel data at allmay be supplied -instead, each tank 50, 52, 53 may be identified as aground-use tank 52 or a standard-use tank 50, 53, and such examples mayrely on the tanks 50, 52, 53 being correctly filled accordingly.

The power system 4 comprises an adjustable fuel delivery system 220,allowing a selection to be made of which tank(s) 50, 52, 53, andtherefore what fuel or fuel blend, to use. A fuel manager 214 asdescribed above may control this system 220. In examples in which thefuel blend or fuel can be changed in flight (rather than having oneflight setting and one ground setting), the fuel characteristics mayvary over the course of a flight – a specific fuel or fuel blend may beselected to improve operation at certain flight stages or in certainexternal conditions. In some examples, calorific values for eachavailable fuel may be calculated or provided, and fuel supply in flightmay be controlled accordingly.

In implementations in which the fuel from the first fuel tank 52 issupplied to a main gas turbine engine 10 of the aircraft 1, whichprovides propulsive power to the aircraft 1, the power system 4 may bereferred to more specifically as a propulsion system 2. The broader term“power system” 4 is used to ensure that implementations in which thefuel is additionally or alternatively supplied to an APU 44 arecaptured, as propulsive power may not be provided by such power systems4, as discussed above.

A method 2080 of operating an aircraft 1 comprising one or more gasturbine engines 10, 44 and a plurality of fuel tanks 50, 52, 53 arrangedto store fuel to power the gas turbine engine(s) 10, 44 is shown in FIG.20 .

The method 2080 comprises providing 2082 at least two fluidly-separatefuel tanks 50, 52, 53 – i.e. at least two fuel sources.

The method 2080 further comprises 2086 controlling fuel input to the gasturbine engine(s) 10, 44 so as to use fuel only from the first fuel tank52 when the aircraft 1 is performing at least the majority of operationson the ground. Here, using fuel from the first tank 52 for “at least themajority” of operations may mean that at least 90% or 95% of the fuelused for ground-based operations is fuel from the first tank 52, and/orat least 90% or 95% of the operation time for ground-based operations ispowered by fuel from the first tank 52, and/or that fuel from the firsttank 52 is used for all ground-based operations except initial enginestart-up. The fuel manager 214 may therefore use fuel from the firstfuel tank 52 for some, but not all, ground-based operations. Forexample, the fuel manager 214 may switch to a different fuel if the fuelfrom the first fuel tank 52 is used up before the operations on theground are complete, and/or the fuel manager 214 may supply the fuelfrom the first fuel tank 52 to the APU 44 but a different fuel to theother gas turbine engine(s) 10. Optionally only fuel from that tank 52may be used for all ground-based operations.

Optionally, the method 2080 may comprise taking fuel from only the oneor more secondary fuel tanks 50, 53 for other operations. In otherexamples, fuel from the first fuel tank 52 may be provided in flight aspart of a blend, but optionally not alone (except in emergencysituations).

In some examples, one or more of the fuel tanks 50, 52 may be part of aseparate set of interlinked fuel tanks. In other examples, each fueltank 50, 52 may be a stand-alone, single-tank, fuel source.

The method 2080 of some examples further comprises storing 2084information on the fuel contained in each fuel tank 50, 52, 53 and/oridentifying each fuel tank 50, 52, 53, optionally in memory of anonboard fuel manager 214. The information stored may simply be a flag asto whether or not a particular tank 50, 52 is the first tank 52 / isintended for use for ground-based operations. Additional information maybe stored in other examples. The controlling step 2086 may be performedbased on this stored information.

A propulsion system 2, or other power system 4, for an aircraft 1 maytherefore comprise a fuel manager 214 arranged to control fuel input tothe gas turbine engine(s) 10, 44 so as to take fuel from the first fueltank 52 when the aircraft 1 is performing ground-based operations, andto take fuel from the one or more secondary fuel tanks 50, 53 for otheroperations. In some implementations, the fuel manager 214 may bearranged to control fuel input to the gas turbine engine(s) 10, 44 so asto take fuel from only the first fuel tank 52 when the aircraft 1 isperforming ground-based operations, and to take fuel from only the oneor more secondary fuel tanks 50, 53 for other operations.

In some examples, the fuel manager 214 may be arranged to storeinformation on each tank / on the fuel contained in each fuel tank 50,52, 53 and to control fuel input to the gas turbine engine(s) 10, 44 inoperation accordingly. The information on the fuel contained in eachfuel tank 50, 52, 53 may simply comprise a flag for whether or not eachtank is the/a ground-use- tank. In such examples, which tank of theplurality of fuel tanks 50, 52, 53 is the first tank 52 may vary overthe lifetime of the propulsion system 2, for example depending on whichtank is filled with which fuel. In other examples, the fuel deliverysystem 220 may be set up for one specific tank 52 to always be theground-use tank, and no such information may need to be stored. Inexamples in which fuel information is stored, the information on thefuel contained in each fuel tank 50, 52, 53 may additionally comprisemore information, such as a %SAF content for each tank, and/or one ormore other fuel characteristics of the fuel currently in each tank 50,52, 53.

The fuel manager 214 may be provided as part of a fuel delivery system220 arranged to allow control and adjustment of the fuel supplied to thegas turbine engine 10, 44, and may be as described above.

In the example shown in FIG. 21 , the fuel management system 220 isarranged to allow fuels from the second and third fuel sources 50, 53 tobe mixed, so as to form a blend for supply to the engine 10, but doesnot allow fuel from the first, ground-use, tank 52 to be blended withthe other fuels. Different approaches may be used in other examples.

The fuel manager 214 of the examples currently being described isarranged to take fuel exclusively from the first fuel tank 52 forground-based operations of the power system 4. It will be appreciatedthat whilst the aircraft 1 technically still has one or more wheels onthe ground for the majority of take-off, take-off is a relativelyhigh-powered activity and is generally classed as part of the flightenvelope, not as a ground-based operation.

Optionally, the fuel manager 214 may additionally receive other data (inaddition to information denoting which tank 50, 52, 53 is the first tank52, and other optional fuel characteristic data), and use that otherdata and the fuel characteristic data to determine a desired fuelcomposition for the gas turbine engine 10, 44 in flight.

As mentioned above, the fuel manager 214 may be provided as a separatefuel management unit 214 built into the propulsion system 2, and/or assoftware and/or hardware incorporated into the pre-existing aircraftcontrol systems. In some examples, fuel composition data and/or tankidentification data may be stored separately from the circuitryperforming the fuel supply management and be retrieved when required –wherever the data are stored, that storage can be thought of as a partof the fuel manager 214, whether or not it is integral or physicallyconnected in any way.

The fuel manager 214 of the examples currently being described isarranged to control fuel input to the gas turbine engine 10, 44 so as totake fuel from the first fuel tank 52 to power at least the majority ofoperations on the ground.

In some examples, the fuel manager 214 may be arranged to be able to mixfuels from the secondary fuel tanks 50, 53 arranged to power the gasturbine engine 10 in flight, but not to be able to mix fuel from thefirst fuel tank 52 with fuel from the secondary fuel tanks 50, 53.

In some examples, the fuel manager 214 may be arranged to select aspecific fuel or fuel combination from one or more of the plurality offuel tanks based on thrust demand of the gas turbine engine in flight.In particular, a fuel with a lower calorific value may be supplied tothe gas turbine engine 10 at lower thrust demand, and vice versa. A fuelwith a higher calorific value may therefore be used at high-power stagesof the flight envelope, such as during take-off. The fuel manager 214may be arranged such that a fuel with a lower calorific value issupplied to the gas turbine engine 10 at cruise than during climb.Optionally, a fuel with a still lower calorific value may be supplied tothe gas turbine engine 10 at ground idle (low idle).

The fuel manager 214 may make changes to the fuel supply directly, ormay provide a notification or suggestion to the pilot regarding thechange, for approval, as described in more detail above. In someexamples, the same fuel manager 214 may automatically make some changes,and request others, depending on the nature of the change.

In some examples, an aircraft 1 may be modified to perform the method2080 described above, optionally by installing an adjustable fueldelivery system 220.

A method 2081 of modifying an aircraft 1 in such a way is shown in FIG.22 . The original aircraft 1 comprises a gas turbine engine 10, and inthis example the gas turbine engine 10 comprises an engine core 11comprising a turbine 19, a compressor 14, and a core shaft 26 connectingthe turbine to the compressor. The aircraft 1 also comprises a fan 23located upstream of the engine core, the fan comprising a plurality offan blades and being arranged to be driven by an output from the coreshaft. The original aircraft 1 may further comprise an APU 44, the APUitself being or comprising a gas turbine engine 44.

The method 2081 comprises providing 2083 at least two separate (notfluidly-linked, or at least capable of being fluidly isolated from eachother) fuel tanks 50, 52, 53, arranged to be able to supply fuel to theengine 10. A first fuel tank 52 of the plurality of fuel tanks 50, 52,53 is arranged to be used to provide fuel for at least the majority ofground-based operations. One or more other fuel tanks 50, 53 arearranged to be used to provide fuel for all operations not covered bythe first fuel tank 52.

In some cases, the aircraft 1 may already comprise a plurality of fueltanks 50, 52, 53 arranged to store fuel to power one or more gas turbineengines 10, 44; in such examples, the step 2083 of arranging the fueltanks may simply comprise ensuring that one of the tanks 52 is set up tobe used for ground-based operations and that at least one different tank50, 53 is set up to be used in flight.

In cases in which the aircraft 1 previously only had a single fuel tank50, a new fuel tank 52 may be added so as to provide a plurality of fueltanks. In cases in which the aircraft 1 previously only had a singlefuel source, albeit comprised of multiple tanks, a new fuel tank 52 maybe added and/or fuel lines may be adjusted such that the original tanks50, 53 are no longer all fluidly interconnected, so providing at leasttwo separate fuel sources. The arranging fuel tanks / providing aseparate first fuel tank 52 step 2083 may therefore vary depending onthe initial aircraft configuration.

The method 2081 further comprises providing 2085 a fuel manager 214arranged to control fuel input to the gas turbine engine(s) 10, 44 so asto use fuel only from the first fuel tank 52 to power at least themajority of aircraft operations on the ground. The fuel manager 214 mayalso be arranged to store information on the fuel contained in each fueltank 50, 52, 53, and/or an identifier for each tank to indicate itsintended use.

The storage and control functions may be performed by separate entitiesor the same entity; it will be appreciated that the fuel manager 214 maytherefore be a distributed system or a single unit or module, asdescribed above. The step of providing 2085 the fuel manager 214 maycomprise or consist of installing software in extant memory, to beexecuted using extant systems, in some examples. In other examples, anew physical unit or module may be mounted onto the propulsion system 2(or other power system 4), optionally including one or more flowcontrollers 216 and/or replacement fuel line sections as appropriate toachieve the desired fuel flow and mixing control.

In some examples, the fuel manager 214 may be additionally arranged toperform other functions, for example, the fuel manager 214 mayadditionally be arranged to control fuel input to the gas turbine engine10 in flight by selection of a specific fuel or fuel combination fromone or more of the plurality of fuel tanks 50, 53 based on thrust demandof the gas turbine engine 10 such that a fuel with a lower calorificvalue is supplied to the gas turbine engine 10 at lower thrust demand,and vice versa. It will be appreciated that thrust demand may bedetermined using any one or more approaches known in the art, forexample based on fuel flow rate and/or power lever angle in the cockpitor one or more other pilot settings, and optionally taking account ofoutside air density, or a proxy for it such as altitude, ambienttemperature, and/or pressure. The examples currently being described maytherefore be used in conjunction with examples described above.

The inventors also appreciated that whilst use of SAF may providebenefits over many parts of the flight envelope, initial engine start-up(i.e. from when the engine is “cold” / not in operation whilst anaircraft 1 is parked) may suffer due to certain properties of some SAFtypes, such as potentially increased viscosity and/or lower lubricity ofSAF as compared to more traditional, fossil-derived, fuels. Changes incontrol of the aircraft’s fuel system may therefore allow technical andenvironmental benefits of SAF to be realised without compromisingstart-up operation. In particular, using a short “burst” of afossil-based hydrocarbon fuel, such as Jet A, Jet A-1, or another fueloptimised for use in cold-start / start-up conditions (SAF orotherwise), for start-up before switching to a SAF fuel (or to adifferent SAF fuel) may provide smoother start-up whilst still allowingthe benefits of SAF to be obtained thereafter. More generally, whateverfuel is to be used later in operation, a fuel optimised for start-up maybe used initially - such a fuel may have a lower freeze temperatureand/or a lower viscosity at a given temperature than other fuel aboardthe aircraft 1.

Whilst it will be appreciated that a synthetic fuel could be made toexactly mimic a traditional kerosene fuel, one or more fuelcharacteristics of SAF stored onboard the aircraft may differ from thefuel characteristics of the one or more other, fossil-derived, fuelsstored onboard the aircraft, in other tanks. In particular, theviscosity of SAFs may be higher at a given temperature than that for atraditional fossil-derived fuel, and decrease with increasingtemperature. As such, once the engine has warmed up, such a fuel willhave an improved viscosity for use due to the increased temperature.

A fuel optimised for start-up – for example having a higher lubricity ata given temperature and/or a lower heat capacity – may therefore beselected, which may be of particular benefit for start-up in coldconditions, irrespective of whether or not any or all fuels carried areor include SAF. The start-up optimised fuel may or may not be certifiedfor use in flight - it may be dedicated to start-up use.

A first fuel source and a second fuel source may therefore be used,providing different first and second fuels. The second fuel may beselected to have improved characteristics with respect to start-upoperation. In some cases, the first fuel may be a sustainable aviationfuel and the second fuel may be fossil-derived; however, this option isnot intended to be limiting.

In the present examples, described with respect to FIGS. 14 and 18 , thefirst fuel source is the first fuel tank 52. In other examples, thefirst fuel source may comprise multiple interlinked tanks. In someexamples, the first fuel tank 52 is arranged to contain only a fuelwhich is a pure sustainable aviation fuel (SAF), i.e. 100% sustainablysourced and not kerosene derived / of fossil origin. In other examples,multiple fuel tanks of a plurality of fuel tanks may all contain SAF –any one of the subset of fuel tanks containing SAF may therefore be usedto supply SAF, or the first fuel tank 52 may contain a SAF-blend or afossil-derived fuel; it will be appreciated that the example of just onefuel tank 52 containing SAF is described here by way of non-limitingexample only. In the present examples, described with respect to FIGS.14 and 18 , the second fuel source is or comprises the second fuel tank50. The second fuel tank 50 is arranged to contain only a fuel which isselected for improved start-up properties, and which may befossil-derived / petroleum-based. Again, it will be appreciated thatthese arrangements are being described by way of example only, and arenot intended to be limiting.

In some implementations, the first fuel tank 52 and the second fuel tank50 may be used to supply fuel to both the main gas turbine engine(s) 10and the APU 44. In other implementations, either or both tanks 50, 52may be used to supply fuel to either the main (propulsive) gas turbineengine(s) 10 or the APU 44, but not both – other fuel tanks may beprovided to provide fuel to the other gas turbine engine(s) 10, 44 insuch implementations.

In some of the examples being described, all fuel used for groundoperations except for that used for the initial start-up is sustainableaviation fuel or a high %SAF blend, and all other fuel used for groundoperations is therefore taken from the first fuel tank 52 containing thesustainable aviation fuel or high %SAF blend (in examples with multipleSAF-containing tanks, any one or more of those tanks may be used, asappropriate). The fuel used for start-up may be SAF or a SAF blend insome cases.

A fuel manager 214 as described above may be arranged to control fuelinput to the gas turbine engine(s) 10, 44 so as to take fuel from thesecond fuel tank 50 at start-up, and then to switch to another fuel tank52.

SAF or a high %SAF blend may therefore be used when the aircraft isperforming at least the majority of operations on the ground (as definedabove), so optionally providing one or more of the benefits describedabove (e.g. reduced nvPM), whether or not the fuel used for the initialengine start-up is SAF. In some implementations, SAF or SAF-blends maybe used for the majority of, or all, aircraft operations, bothground-based and flight.

Although FIGS. 14 and 18 show the second fuel tank 50 as relativelylarge, in some implementations, the second fuel tank 50 may be smallerthan the one or more other fuel tanks 52, 53. For example, the firstfuel tank 50 may represent 1% or 2% to 15 %, and optionally 3% to 5%, ofthe total available tank volume of the aircraft 1. Optionally, that tank50 may be arranged to be used exclusively for engine start-up. A fueloptimised for start-up use, even at the cost of performance in normaluse, may therefore be selected as the second fuel.

Each fuel tank 50, 52 onboard the aircraft 1 may be arranged to containa fuel of a different type (e.g. petroleum-origin fuel or SAF, ordifferent SAF varieties), and some tanks may contain blended fuels witha proportion of a sustainable aviation fuel mixed with a traditional jetfuel or other petroleum-origin fuel. At least one tank 52 may containSAF – i.e. purely a sustainable aviation fuel, not a blend, in someexamples. At least one tank 50 contains a start-up optimised fuel, whichmay or may not be a fossil-based fuel.

The propulsion system 2 of the examples being described again comprisesan adjustable fuel delivery system 220, allowing a selection to be madeof which tank(s) 50, 52, 53, and therefore what fuel or fuel blend, touse. In such examples, the fuel characteristics may vary over the courseof a journey -a specific fuel or fuel blend may be selected to improveoperation at certain flight stages or in certain external conditions,for example as described above with respect to other aspects.

In examples in which detection is used for one or more fuelcharacteristics (either by direct detection, or by inference fromdetected parameters), e.g. to discover or verify which tank contains thefuel for use at start-up, any of the detection approaches describedabove may be implemented. In other examples, no detection may beperformed and supplied data on fuel composition may be relied uponinstead - that data may be simply e.g. “Fuel for start-up” or “Other”,or may include more detailed fuel characteristic information. In otherexamples, no fuel data at all may be supplied – instead, each tank 50,52, 53 may be identified as e.g. a “Start-up” tank or a “Normal use”tank, and the example may rely on the tanks 50, 52, 53 being correctlyfilled accordingly.

In some examples, calorific values for each available fuel may becalculated or provided, and a fuel or fuel blend supplied based onthrust demand as described above (optionally also consideringaltitude) - some fuel from the first tank 52 (e.g. SAF) and/or fuel fromthe second tank 50 (e.g. a fossil-based fuel) may be used alone and/orin one or more blends in such examples.

A method 2021 of operating an aircraft 1 comprising a gas turbine engine10, 44 and a plurality of fuel tanks 50, 52 arranged to store fuel topower the gas turbine engine 10, 44 is shown in FIG. 23 .

The method 2021 comprises arranging 2023 two fuel tanks 50, 52 of theplurality of fuel tanks to each store a different fuel. In particular, afirst fuel tank 52 of the plurality of fuel tanks is arranged to containonly a fuel which is a sustainable aviation fuel in this example, and asecond fuel tank 50 of the plurality of fuel tanks is arranged tocontain a fuel selected for improved start-up properties, which may beor comprise a fossil-based hydrocarbon fuel. This arranging step 2023may comprise fluidly isolating one or more tanks from each other so asto allow different fuels to be stored in different tanks (e.g. byclosing valves). This arranging step 2023 may comprise filling the tanksappropriately.

In some examples, one or more of the fuel tanks 50, 52 may be part of aseparate set of interlinked fuel tanks. In other examples, each fueltank 50, 52 may be a stand-alone, single-tank, fuel source.

The method 2021 further comprises controlling 2025 fuel input to the gasturbine engine 10 so as to take fuel from the second tank 50 for enginestart-up, before switching to the first fuel tank 52. The first fueltank 52 may contain SAF, and may be the only fuel source used forground-based operations after start-up, such that SAF is used when theaircraft 1 is performing at least the majority of operations on theground.

The control 2025 of the fuel, optionally managed by a fuel manager 214,may comprise switching from taking fuel from the second tank 50 totaking fuel from the first fuel tank 52 when a selected parameter, suchas fuel temperature, turbine gas temperature, oil temperature, shaftspeed, or time since the engine was turned on, reaches a certainthreshold. For example, the change in fuel may be made when:

-   (i) the fuel reaches a temperature of at least 60° C., and    optionally of 80° C., 85° C., 90° C., 95° C., or 100° C., at the    inlet to the combustor 16; or-   (ii) after the gas turbine engine 10, 44 has been running for a    period of at least thirty seconds, at least one minute, at least    three minutes, or at least five minutes. For some engines 10, a    period of at least 10 minutes or 15 minutes may be selected.

It will be appreciated that a suitable time period may depend onenvironmental conditions (e.g. a lower air temperature and correspondinglower initial fuel temperature may result in a longer start-up timeperiod) and on properties of the aircraft 1 and fuel supply system, andmay be adjusted as applicable for a given aircraft 1 and environment.

In some implementations, the switch from taking fuel from the secondtank 50 to taking fuel from the first fuel tank 52 may be actioned whenthe engine 10 reaches idle conditions, and optionally a short period(e.g. thirty seconds) after idle conditions have been reached to ensurethat the idle operation has stabilised. In some cases, the engine 10 maybe allowed to run at idle for at least two minutes, or at least fiveminutes. In some implementations, a time period may be set based on forhow long the engine 10 has been shut down since its last use – forexample setting the period at two minutes if the engine 10 has been shutdown for less than 90 minutes, and at five minutes if the engine hasbeen shut down for longer than 90 minutes.

The time at which idle operation is reached may be identified based on atemperature or shaft speed, for example, which may be specific to theengine 10 and/or aircraft 1 in question.

In some implementations, the switch from taking fuel from the secondtank 50 to taking fuel from the first fuel tank 52 may be actioned whenthe engine 10 reaches a defined limitation in the Engine OperatingInstructions that prevents take-off until certain criteria are met. Itwill be appreciated that specific parameters and values can be looked upin the Engine Operating Instructions for a given engine 10. The engine10 reaching a state in which it would be ready for take-off indicatesthat a start-up phase is complete (although the change of fuel may bemade earlier in some implementations).

The method 2021 optionally further comprises storing 2027 information onthe fuel contained in each fuel tank 50, 52, optionally in memory of anon-board fuel manager 214. The information stored may simply be a flagas to whether or not a particular tank 50, 52 contains a fuel selectedfor its start-up properties (which may be fossil-based). A flag markingone or more tanks as containing 100% SAF or a high %SAF blend may alsobe provided. Additional information may be stored in other examples.This stored information may be used for the controlling step 2025, andin particular may be used to identify the first fuel tank 52 (and/orcorrespondingly one or more tanks containing fuel for general use / usefor purposes other than start-up, if there are multiple such tanks) andthe second fuel tank 50 (and/or correspondingly one or more other tanks,if multiple tanks contain fuels suitable for start-up), if that is nothard-coded / hard-wired into the propulsion system 2.

A power system 4 for an aircraft 1 may therefore comprise a fuel manager214 arranged to store information on each fuel tank 50, 52, 53 / on thefuel contained in each fuel tank 50, 52, 53 and to control fuel input tothe gas turbine engine 10, 44 in operation. The information stored maysimply comprise a flag for whether or not each tank is the start-uptank, or may comprise more detailed information, such as a %SAF contentfor each tank, and/or one or more other fuel characteristics of the fuelcurrently in each tank 50, 52. In such examples, which tank 50, 52 ofthe plurality of fuel tanks 50, 52, 53 is the first tank 52 and which isthe second fuel tank 50 may vary over the lifetime of the power system4, for example depending on which tank is filled with which fuel. Inother examples, the fuel delivery system 220 as shown in FIG. 16 may beset up for one specific tank 50 to always be the start-up tank, and nosuch information may need to be stored. Optionally, one specific tank 52may always be a/the SAF tank or high %SAF blend tank.

In implementations in which the start-up fuel and the other, first, fuelare supplied to a main gas turbine engine 10 of the aircraft 1, whichprovides propulsive power to the aircraft 1, the power system 4 may bereferred to more specifically as a propulsion system 2. The broader term“power system” 4 mentioned above is used here to ensure thatimplementations in which the fuels are additionally or alternativelysupplied to an APU 44 are captured, as propulsive power may not beprovided by such power systems 4.

As described above for other examples, the fuel manager 214 may beprovided as part of a fuel delivery system 220 arranged to allow controland adjustment of the fuel supplied to the gas turbine engine 10, 44;any features described above may be applied as appropriate to theexamples currently being described. In examples with APUs 44, the fuelmanager 214 may be arranged to control the fuel or fuel blend providedto the APU 44 as well as to the main, propulsive, engine(s) 10.

The APU 44 may be required in flight, in some circumstances, and itsavailability under such circumstances may be time-critical – for exampleto re-start the main engines 10 after a main engine flame-out. Whenstarting up the APU 44 in flight, it may be cold (having been unused forup to several hours); the fuel manager 214 may therefore be(alternatively or additionally) arranged to provide the second fuel toperform the start-up of the APU 44 in flight quickly and reliably. Toavoid losing time by inadvertently trying to start the APU 44 on a fuelwith poor properties for start-up, the APU 44 could be automaticallyswitched to draw fuel from the second tank (e.g. with a lower-viscosityfuel at a given temperature) whenever the aircraft 1 is airborne (e.g.with reference to the “weight-on-wheels” indicator), but to use any fuelwhen on the ground (when start-up time is less likely to be crucial).

In some implementations, the first tank 52 may be empty for at leastpart of the airborne part of the journey, or indeed refilled with adifferent fuel if the tank 52 is used as a trim tank. The fuel manager214 may therefore be arranged to take appropriate action depending oncurrent content and/or usage of the first fuel tank 52 when consideringwhether or not to take fuel from the first fuel tank for engine 10, 44start-up in flight.

As described above for other examples, the fuel manager 214 mayadditionally receive other data (in addition to information denotingwhich tank(s) contain(s) a fuel selected for start-up, and otheroptional fuel characteristic data, such as SAF content), and use thatother data and the fuel characteristic data to determine a desired fuelcomposition for the gas turbine engine 10, 44 in flight.

In some examples, an aircraft 1 may be modified to perform the method2021 described above, optionally by installing an adjustable fueldelivery system 220.

A method 2031 of modifying an aircraft 1 in such a way is shown in FIG.24 . The original aircraft 1 comprises a gas turbine engine 10, the gasturbine engine 10 optionally comprising an engine core 11 comprising aturbine 19, a compressor 14, and a core shaft 26 connecting the turbineto the compressor. The aircraft 1 also comprises a fan 23 locatedupstream of the engine core, the fan comprising a plurality of fanblades and being arranged to be driven by an output from the core shaft.The original aircraft 1 may further comprise an APU 44, the APU itselfbeing or comprising a gas turbine engine 44.

The method 2031 comprises arranging 2033 two fuel tanks 50, 52 of theplurality of fuel tanks to each store a different fuel. In particular, afirst fuel tank 52 of the plurality of fuel tanks is arranged to containa first fuel, which may be a sustainable aviation fuel, and a secondfuel tank 50 of the plurality of fuel tanks is arranged to contain afuel selected for its improved start-up properties as compared to thefirst fuel, and which may be a fossil-based hydrocarbon fuel.

In some cases, the aircraft 1 may already comprise a plurality of fueltanks 50, 52 arranged to store fuel to power the gas turbine engine(s)10, 44; in such examples, the step 2033 of arranging the fuel tanks maysimply comprise filling the tanks selectively with different fuels. Incases in which the aircraft 1 previously only had a single fuel tank 50,one or more new fuel tanks 52, 53 may be added so as to provide aplurality of fuel tanks. In cases in which the aircraft 1 previouslyonly had a single fuel source, albeit comprised of multiple tanks, a newfuel tank 52 may be added and/or fuel lines may be adjusted such thatthe original tanks 50, 53 are no longer all fluidly interconnected, soproviding at least two separate fuel sources. The arranging step 2033may therefore vary depending on the initial aircraft configuration.

The method 2031 further comprises providing 2035 a fuel manager 214arranged to control fuel supply so as to take fuel from the second tank50 for engine start-up, before switching to the first fuel tank 52. Thefuel manager 214 may use fuel from the first fuel tank 52 (which is SAFin a specific example described, but may be a SAF blend or a purefossil-based fuel in other examples) for all ground-based operationsafter start-up.

In some examples, such as arrangements in which the tank(s) used tostore the fuel selected for start-up may vary over the lifetime of thepropulsion system 2, the fuel manager 214 may additionally be arrangedto store information on the fuel contained in each fuel tank 50, 52 soas to allow the first tank 52 and second tank 50, or equivalent(s), tobe identified.

The storage and control functions may be performed by separate entitiesor the same entity; it will be appreciated that the fuel manager 214 maytherefore be a distributed system or a single unit or module. The stepof providing 2035 the fuel manager 214 may comprise or consist ofinstalling software in extant memory, to be executed using extantsystems, in some examples. In other examples, a new physical unit ormodule may be mounted onto the propulsion system 2, optionally includingone or more flow controllers 216 and/or replacement fuel line sectionsas appropriate to achieve the desired fuel flow and mixing control.

In some examples, the fuel manager 214 may be additionally arranged toperform other functions, for example to control fuel input to the gasturbine engine 10 by selection of a specific fuel or fuel combinationfrom one or more of the plurality of fuel tanks 50, 52, 53 based onthrust demand of the gas turbine engine 10 such that a fuel with a lowercalorific value is supplied to the gas turbine engine 10 at lower thrustdemand, and vice versa. It will be appreciated that thrust demand may bedetermined using any one or more approaches known in the art, forexample as mentioned above.

The inventors also appreciated that, as different fuels can havedifferent properties whilst still conforming to the standards, knowledgeof the flight profile can allow a selection to be made of which of thefuels available to an aircraft 1 is used for which portion(s) of theflight profile (when multiple fuels are available) - this may provideimproved aircraft performance. For example, a fuel with improvedemissions outcomes may be selected for operations at or near an airport,and a fuel with a higher calorific value may be used for operations withhigher thrusts. A fuelling schedule defining which fuel, or fuel blend,to use for each portion of the flight profile may therefore bedetermined based on knowledge of the flight profile and of the availablefuels.

Such a method 3020 is depicted in FIG. 25 . It will be appreciated thatan aircraft 1 for which the method 3020 is implemented must comprise atleast two fuel sources 50, 53, such that at least two different fuels(i.e. fuels with at least one difference in fuel characteristics betweenthem) are available for use. It will be appreciated that the use of fuelblends with different blend ratios may allow for many more than twofuelling options even with only two different fuels stored onboard.

The method 3020 may be performed on-wing, e.g. by a fuelling scheduledetermination module 250 of the aircraft 1 as illustrated in FIG. 21 .Such a fuelling schedule determination module 250 may form a part of anelectronic engine controller (EEC) 42 of the aircraft 1, optionallybeing provided as software installed on an extant EEC 42, or added as amodule thereto, or may be provided by a separate module. Alternatively,the method 3020 may be performed off-wing, and the fuelling scheduleprovided to the aircraft 1 for implementation. It will be appreciatedthat any suitable processing means may be used to perform the role ofthe fuelling schedule determination module 250, and thatcomputer-readable instructions to cause the processing means toimplement the method 3020 being described may be provided.

The method 3020 may therefore be performed by processing circuitry ofthe aircraft 1, or by separate, off-wing, processing circuitry. Themethod 3020 may be performed by processing circuitry of a portablecomputing device, for example a pilot’s personal computing device.

A fuel manager 214 as described above may be used to implement thefuelling schedule. The fuelling schedule determination module 250, orother processing circuitry, may provide the fuelling schedule to thefuel manager 214 for implementation.

The method 3020 comprises obtaining a flight profile for a flight of theaircraft 1. The flight profile may be provided to a fuelling scheduledetermination module 250 of the aircraft 1, or to an off-wing fuellingschedule determination module. The flight profile may be obtained in anysuitable way, for example being sent electronically, manually enteredvia a user interface, or retrieved from memory.

The method 3020 further comprises determining 3024 a fuelling schedulefor the flight of the aircraft 1 based on the flight profile and thefuel characteristics of the available fuels. In determining 3024 thefuelling schedule, an amount of each fuel available onboard the aircraft1 is also taken into account. The altitude and route of the aircraft 1for the intended flight are defined in the flight profile. Expectedthrust demands may be included in the flight profile, or determined orinferred based on the flight profile, and may be used to guide fuelselection. In addition, data relating to forecast weather conditionsalong the intended route of the aircraft 1 as defined in the flightprofile may be provided with the flight profile, or requested based onthe flight profile. The weather data may be used to influence thedetermination 3024 of fuel scheduling.

The determined fuelling schedule specifies a desired variation with timeof how much fuel is drawn from each tank 50, 53; i.e. it lists whichfuel or fuel blend should be used for each stage of the flight asdefined in the flight profile, and therefore determines when a change infuelling should be made, and the nature of the change.

The fuel characteristics considered comprise one or more of the fuelcharacteristics as defined elsewhere herein.

For example, the amount of sustainable aviation fuel – SAF – availableto the aircraft 1, optionally as both pure SAF and in blends, may bedetermined. As described above, using SAF or high %SAF blends for groundoperations may offer reduced nvPM emissions and therefore improvedairport air quality, and the fuelling schedule may prioritise the use ofSAF / a fuel with a high %SAF for ground-based operations of theaircraft 1.

Similarly, and as described above, a calorific value of each fuelonboard the aircraft 1 may be a fuel characteristic of interest, and thefuelling schedule may prioritise the use of higher calorific value fuelsfor higher thrust operations of the aircraft 1, and of lower calorificvalue fuels for lower thrust operations of the aircraft 1.

The fuelling schedule may be determined onboard the aircraft 1; forexample in an engine electronic controller 42 or other processingcircuitry of the aircraft 1, or in a device belonging to the pilot.Alternatively, the fuelling schedule may be determined off-wing, e.g. ina ground-based server or other computing system. The determined fuellingschedule may therefore be provided 3025 to the aircraft 1 prior to, orat the start of, the flight to which the fuelling schedule applies.Different steps of the method 3020 may therefore be performed byentirely separate entities in some cased, or all within or by theaircraft 1 in other cases.

In some implementations, the method 3020 further comprises controlling3026 fuel input to the gas turbine engine 10 in operation in accordancewith the fuelling schedule. In particular, a fuel manager 214 asdescribed above may receive the fuelling schedule and control fuelsupply accordingly, for example by opening or closing one or morevalves, or activating or deactivating one or more pumps, as appropriateto provide the desired fuel or fuel blend at each stage of the flight.

A propulsion system 2 for an aircraft 1 may be arranged to implement themethod 3020 described above. The propulsion system 2 comprises a gasturbine engine comprising at least two separate fuel sources 50, 53,such that at least two different fuels (i.e. fuels having different fuelcharacteristics) are stored aboard the aircraft 1.

The propulsion system 2 of such examples includes a fuelling scheduledetermination module 250 which is arranged to obtain 3022 a flightprofile for a flight of the aircraft 1; and determine 3024 a fuellingschedule for the flight based on the flight profile and the fuelcharacteristics.

The fuelling schedule determination module 250 may implement the control3026 of fuel supply to the gas turbine engine 10 itself, or may pass thefuelling schedule to a fuel manager 214 for implementation.

The propulsion system 2 of some examples additionally comprises areceiver 251 arranged to receive forecast weather conditions for theintended route of the aircraft 1, which is defined in the flightprofile. The received forecast weather conditions may be used toinfluence the fuelling schedule, as mentioned above.

It will be understood that the invention is not limited to theembodiments above-described and various modifications and improvementscan be made without departing from the concepts described herein. Exceptwhere mutually exclusive, any of the features may be employed separatelyor in combination with any other features and the disclosure extends toand includes all combinations and subcombinations of one or morefeatures described herein.

We claim:
 1. A method of operating an aircraft comprising a propulsionsystem, the propulsion system comprising a gas turbine engine and a fueltank arranged to provide fuel to the gas turbine engine, the methodcomprising: determining at least one fuel characteristic of the fuelarranged to be provided to the gas turbine engine; and controlling thepropulsion system based on the at least one fuel characteristic.
 2. Themethod of claim 1, wherein the at least one fuel characteristic of thefuel comprises at least one of: i. percentage of sustainable aviationfuel in the fuel; ii. aromatic hydrocarbon content of the fuel; iii.multi-aromatic hydrocarbon content of the fuel; iv. percentage ofnitrogen-containing species in the fuel; v. presence or percentage of atracer species or trace element in the fuel; vi. hydrogen to carbonratio of the fuel; vii. hydrocarbon distribution of the fuel; viii.level of non-volatile particulate matter emissions on combustion; ix.naphthalene content of the fuel; x. sulphur content of the fuel; xi.cycloparaffin content of the fuel; xii. oxygen content of the fuel;xiii. thermal stability of the fuel; xiv. level of coking of the fuel;xv. an indication that the fuel is a fossil fuel; and xvi. at least oneof density, viscosity, calorific value, and heat capacity.
 3. The methodof claim 1, further comprising receiving weather data relating toweather conditions around the aircraft or along a planned route of theaircraft, and wherein the received weather data are used to influencethe control of the propulsion system.
 4. The method of claim 1, furthercomprising detecting weather conditions around the aircraft in flight,and wherein the detected weather conditions are used to influence thecontrol of the propulsion system.
 5. The method of claim 1, wherein thedetermining the at least one fuel characteristic is performed based ondetection of at least one fuel property.
 6. The method of claim 5,wherein the detection is performed on-wing.
 7. The method of claim 1,wherein the determining the at least one fuel characteristic isperformed based on received fuel composition data.
 8. The method ofclaim 7, wherein the fuel composition data are provided to the aircrafton refuelling.
 9. The method of claim 1, wherein the at least one fuelcharacteristic is determined for fuel in at least one fuel tank of theaircraft.
 10. The method of claim 9, wherein at least one fuelcharacteristic is determined for each of a plurality of fuels stored onboard the aircraft.
 11. The method of claim 1, wherein the at least onefuel characteristic is determined for fuel immediately before entry intoa combustor of the gas turbine engine.
 12. The method of claim 11,wherein the determination of the at least one fuel characteristic offuel immediately before entry into the combustor of the gas turbineengine is performed a plurality of times during flight to account forchanges in fuel composition.
 13. The method of claim 1, wherein at leastone fuel characteristic is inferred from performance of the gas turbineengine during at least one of taxi, take-off and climb of the aircraft,and wherein the propulsion system is controlled during cruise based onthe at least one inferred fuel characteristic.
 14. The method of claim1, wherein the determining at least one fuel characteristic of the fuelcomprises: obtaining at least one fuel characteristic of any fuelalready present in the fuel tank prior to refuelling; determining atleast one fuel characteristic of a fuel added to the fuel tank onrefuelling; and calculating at least one fuel characteristic of theresultant fuel in the fuel tank after refuelling.
 15. A propulsionsystem for an aircraft comprising: a gas turbine engine; a fuel tankarranged to contain a fuel to power the gas turbine engine; a fuelcomposition determination module arranged to: determine at least onefuel characteristic of the fuel arranged to be provided to the gasturbine engine; and a propulsion system controller arranged to: controlthe propulsion system based on the at least one fuel characteristic ofthe fuel.
 16. The propulsion system of claim 15, wherein the fuelcomposition determination module comprises a receiver arranged toreceive data relating to fuel composition, based on which at least onefuel characteristic can be determined.
 17. The propulsion system ofclaim 15, wherein the fuel composition determination module comprises atleast one sensor arranged to provide data relating to at least one fuelcharacteristic.
 18. The propulsion system of claim 15, wherein thepropulsion system comprises a plurality of fuel tanks arranged tocontain different fuels to power the gas turbine engine, and the fuelcomposition determination module is arranged to determine at least onefuel characteristic of each different fuel.